FaceTime Calls from Contacts Only?
Not really.

On OS X, I wasn't able to find any possibility of doing this. I'm eager to be proven wrong on that point, of course.



On iOS, you can kinda do it - by abusing Do Not Disturb. Following these steps will, of course, make it so that your device won't show you notifications. At all.

Really want to do this? Ok, let's get started:


Find your Do Not Disturb preferences. 


iOS 5/6: Settings > Notifications > Do Not Disturb
iOS 7: Settings > Do Not Disturb

Manually turn on Do Not Disturb


iOS 5/6: Root settings window, it's a switch.
iOS 7: In the Do Not Disturb window, there's a 'Manual' switch. Activate it.

Turn off Scheduling:



4. Select 'Allow calls from', and set it to 'All Contacts':



Optionally, disable the two-calls-in-three-minutes feature. I wouldn't though - it's kinda important for emergencies.

Now, your iOS device won't accept FaceTime calls from anyone not in your contacts - at the price of not being able to receive notifications, either.