Preventing bot form submission
This is one problem that a lot of people have encountered. As user166390 points out in the comments, the bot can just submit information directly to the server, bypassing the javascript (see simple utilities like cURL and Postman). Many bots are capable of consuming and interacting with the javascript now. Hari krishnan points out the use of captcha, the most prevalent and successful of which (to my knowledge) is reCaptcha. But captchas have their problems and are discouraged by the World-Wide Web compendium, mostly for reasons of ineffectiveness and inaccessibility.
And lest we forget, an attacker can always deploy human intelligence to defeat a captcha. There are stories of attackers paying for people to crack captchas for spamming purposes without the workers realizing they're participating in illegal activities. Amazon offers a service called Mechanical Turk that tackles things like this. Amazon would strenuously object if you were to use their service for malicious purposes, and it has the downside of costing money and creating a paper trail. However, there are more erhm providers out there who would harbor no such objections.
So what can you do?
My favorite mechanism is a hidden checkbox. Make it have a label like 'Do you agree to the terms and conditions of using our services?' perhaps even with a link to some serious looking terms. But you default it to unchecked and hide it through css: position it off page, put it in a container with a zero height or zero width, position a div over top of it with a higher z-index. Roll your own mechanism here and be creative.
The secret is that no human will see the checkbox, but most bots fill forms by inspecting the page and manipulating it directly, not through actual vision. Therefore, any form that comes in with that checkbox value set allows you to know it wasn't filled by a human. This technique is called a bot trap. The rule of thumb for the type of auto-form filling bots is that if a human has to intercede to overcome an individual site, then they've lost all the money (in the form of their time) they would have made by spreading their spam advertisements.
(The previous rule of thumb assumes you're protecting a forum or comment form. If actual money or personal information is on the line, then you need more security than just one heuristic. This is still security through obscurity, it just turns out that obscurity is enough to protect you from casual, scripted attacks. Don't deceive yourself into thinking this secures your website against all attacks.)
The other half of the secret is keeping it. Do not alter the response in any way if the box is checked. Show the same confirmation, thank you, or whatever message or page afterwards. That will prevent the bot from knowing it has been rejected.
I am also a fan of the timing method. You have to implement it entirely on the server side. Track the time the page was served in a persistent way (essentially the session) and compare it against the time the form submission comes in. This prevents forgery or even letting the bot know it's being timed - if you make the served time a part of the form or javascript, then you've let them know you're on to them, inviting a more sophisticated approach.
Again though, just silently discard the request while serving the same thank you page (or introduce a delay in responding to the spam form, if you want to be vindictive - this may not keep them from overwhelming your server and it may even let them overwhelm you faster, by keeping more connections open longer. At that point, you need a hardware solution, a firewall on a load balancer setup).
There are a lot of resources out there about delaying server responses to slow down attackers, frequently in the form of brute-force password attempts. This IT Security question looks like a good starting point.
Update regarding Captcha's
I had been thinking about updating this question for a while regarding the topic of computer vision and form submission. An article surfaced recently that pointed me to this blog post by Steve Hickson, a computer vision enthusiast. Snapchat (apparently some social media platform? I've never used it, feeling older every day...) launched a new captcha-like system where you have to identify pictures (cartoons, really) which contain a ghost. Steve proved that this doesn't verify squat about the submitter, because in typical fashion, computers are better and faster at identifying this simple type of image.
It's not hard to imagine extending a similar approach to other Captcha types. I did a search and found these links interesting as well:
Is reCaptcha broken?
Practical, non-image based Captchas
If we know CAPTCHA can be beat, why are we still using them?
Is there a true alternative to using CAPTCHA images?
How a trio of Hackers brought Google's reCaptcha to its knees - extra interesting because it is about the audio Captchas.
Oh, and we'd hardly be complete without an obligatory XKCD comic.
No Realy are you still thinking that Captcha or ReCap are Safe ?
Bots nowDays are smart and can easly recognise Letters on images Using OCR Tools (Search for it to understand)
I say the best way to protect your self from auto Form submitting is adding a hidden hash generated (and stored on the Session on your server of the current Client) every time you display the form for submitting !
That's all when the Bot or any Zombie submit the form you check if it the given hash equals the session stored Hash ;)
for more info Read about CSRF !
Your code would not prevent bot submission but its not because of how your code is. The typical bot out there will more likely do an external/automated POST request to the URL (action attribute). The typical bots aren't rendering HTML, CSS, or JavaScript. They are reading the HTML and acting upon them, so any client logic will not be executed. For example, CURLing a URL will get the markup without loading or evaluating any JavaScript. One could create a simple script that looks for <form> and then does a CURL POST to that URL with the matching keys.
With that in mind, a server-side solution to prevent bot submission is necessary. Captcha + CSRF should be suffice. (http://en.wikipedia.org/wiki/Cross-site_request_forgery)
Today I successfully stopped a continuous spamming of my form. This method might not always work of course, but it was simple and worked well for this particular case.
I did the following:
I set the action property of the form to mustusejavascript.asp which just shows a message that the submission did not work and that the visitor must have javascript enabled.
I set the form's onsubmit property to a javascript function that sets the action property of the form to the real receiving page, like receivemessage.asp
The bot in question apparently does not handle javascript so I no longer see any spam from it. And for a human (who has javascript turned on) it works without any inconvenience or extra interaction at all. If the visitor has javascript turned off, he will get a clear message about that if he makes a submission.