How do I keep the ssl key for our website confidential?
A person with administrative (or often even physical) access to a server is going to be able to extract the private key. Whether through exporting, memory sniffing, or other such trickery.
Your administrators have access to the private keys of your web servers. Accept this as fact, and work around that. If your sysadmins aren't trustworthy, then you may need better sysadmins or at least fewer sysadmins with access to the web servers. If it's a matter of management security paranoia, then there may be a deeper issue regarding their ability to trust a sysadmin.
This isn't to say that you should just let everybody have access to the private key. There should always be a need for access before access is granted. With that in mind, are you going to take extreme measures to make sure that a sysadmin with full control of a website can not export the private key, but can still manipulate the website itself in any number of nearly untraceable ways? We're back to trust here, and I think that's the core of the problem that needs to be addressed.
When you import the key, you have the option of marking it as non-exportable. This will prevent you from using IIS or the certificate MMC to export it. At least, it makes it a little harder.
However, if they have an administrator account on the machine, or have physical access to it - they will still be able to get the key through other means.