Found some info here:
Blocking TCP 5223 and the following IP ranges:
Again with the caveat that this will likely take down multiple Apple services as the port is used for the Apple Push Notification Service and the ranges are big.
Also found a blog of someone who studied the Facetime protocol in detail in 2010:
Might have changed since then...
I also found some info on the Aruba website that they at least can identify the conversations to optimise QoS (not with the intention to block anything).
What you might be able to do is a packet capture of DNS queries, and then see if any of them mention FaceTime, and then try blocking those.