There is no way to block individual callers with the FaceTime video chat application. The only solution is to deny or shut down the specific caller when that certain individual initiates a call to you.
Apple does not police these calls, but if you are getting obcene or threatening calls, you would need to call law inforcement or the isp/e-mail service the caller is using and report your problem to them.
@wuzradioman - I am disappointed Apple does not have this feature for facetime. I hope they are listening.
Denying a call or hanging up if I accidentally pick up, is what I have been doing so far. But it is not an acceptable solution. The ringing is too loud and comes at all hours.
Generally I like facetime, but now I have to disable facetime entirely because of one annoying person. (If others are having the same problem, you can disable facetime with: Settings -> General -> Facetime -> Off)
@apple - you could increase your adoption of facetime if you had a mechanism to block these annoying unwanted calls.
If you go to your settings for FaceTime, you will find a section labeled, "You can be reached by FaceTime at:" Your phone number will be there, as will any email address that you have associated with your phone. Uncheck any or all email addresses.
I believe that you will need at least 1 email address in order to contact a person who is using an iPod Touch or iPad. If the address that you use is receiving unwanted FaceTime calls, then you will need to add a different email address which you can be reached, and make sure that the new one is the only address checked in your settings. This also works the same way for iMessage.
Hope this helps!