Apple - Disable the guest user completely in El Capitan without having to disable 'Find My Mac'

@alan-shutko pointed out very important info about this in his comment:

The point is to make Find My Mac work. If you turn Find My Mac off, the guest account will go away. The reason it's there if Find My Mac is on is that if someone steals your mac, it cannot report its position unless the thief connects it to a wifi network.

Looks like we should keep it on.

Ok there is something really broken in the User options in OS X. You can disable the guest account even though FileVault and Find My Mac are on. These are the steps that worked for me:

Step 1. Turn off the Guest User (pay attention, buggy dialog!)

  1. Go to System Preferences - Users & Groups
  2. Unlock all options using the lock on the bottom left
  3. Click on the Guest User and make sure all options are off
  4. If all options were off, enable Allow guests to log in to this computer
  5. Then click on your primary username, then go back to the Guest User and turn off all options again. The Users & Groups dialog is really buggy. It might even automatically check Enable parental controls for no reason.
  6. Before you close the dialog, make sure that all checkboxes are unchecked and that the description of the Guest User says Off.

Step 2. Turn off all Sharing options/services (for now)

  1. Go to System Preferences - Sharing
  2. Uncheck all sharing options/services

Step 3. Disable iCloud Find My Mac (for now)

  1. Go to System Preferences - iCloud
  2. Scroll down the list, and uncheck Find My Mac

Step 4. Manually delete the Guest account

  1. Open up a terminal, and execute the following command:
    sudo dscl . delete /Users/Guest

  2. Check that there is no user Guest anymore with:
    dscl . list /Users

Step 5. Reboot

  1. Make sure that all dialogs are closed
  2. Reboot

If you are lucky, your guest account is gone!
If not, retry step 4 (the dscl command) and reboot again.

Once your guest account is gone, you can re-enable iCloud Find My Mac and your sharing options/services. But stay away from the Users & Groups dialog.

I had the same problem myself and I couldn't find any sources on how to do it. I just found out how to do it without disabling the Find my Mac option. Here's what you have to do. Make sure you disable the Find my mac option. Then click the Apple logo on the top left side then System Preferences > Security & Privacy > General > Advanced > Check "Require an administrator password to access system-wide preferences and after than shut down then open it again. you should see you own account only there without the Guest option. Go to the system preferences again to enable find my mac.