google mail send as accounts.. how to block?

An employee of ours set up his gmail account to send emails as [email protected], which was fine, but now he has left the company and we have disabled his company account.

He is still able to send emails from his gmail account that appear to the recipient as from ourcompany.com. Is there any way I can block this?

Nope, that is not possible. You can use whatever you want in the "From:" header, you could maybe ask google to remove it from his account (you need to confirm your From address in gmail the first time). But there is nothing that stops him from using his old from-address in his desktop-client.

You could setup SPF records to tell remote mail servers that use it that it's not a legit mail.

Is it out of the question to just contact the guy and ask him to change it?

You'll need to find some way to revoke the verification required for him to send emails like that, which would have been part of the initial configuration he did :

http://mail.google.com/support/bin/answer.py?hl=en&ctx=mail&answer=22370

At the very least, you need to get him to remove the option from his account which hopefully would mean he couldn't re-verify it. If you can't manage this, then you'll probably have to contact Google. I'm sure this situation occurs fairly regularly.