I've also noticed that using keywords like face etc doesn't seem to work.
This isn't really possible and it would need to be done via a third party app. Or if you want to block it with a passcode and not time based you can use Apples own built in Parental Controls https://support.apple.com/en-gb/HT201304
Not that I am sure (haven't tried it) but I guess you may be able to do this but it will be pretty hands on.
You will first need to identify the IP addresses/domains that Apple are using for Facetime, you may be able to do this by enabling logging and using facetime to see the destination address requests are being made to.
Once you have the IP(s)/domain(s) you should be able to block them.