In the FaceTime app , then tap Block.
If you receive an unwanted Group FaceTime call from unknown callers, you can block all participants by doing one of the following:
While the call is ringing: Tap the left side of the notification to expand the window, tap , then tap Block All Participants.
Note: If a Group FaceTime call is listed in the Active section, it hasn’t ended; the Block option won’t be available in your call history.
Go to Settings Report a FaceTime call as spam on iPhone