I've never had any FaceTime spam, luckily. Don't use your real email address as your Apple ID. Use something else which is kept secret. Simple as that.
Apple used to allow you to use anything you like as an Apple ID, eg I might have used '7LM' (but didn't, mine was much more obscure, and not an email).
Nowadays I believe, it has to be the primary email associated with the account. ISTR that as you enable certain new iTunes / iCloud features, you may get forced to transition to email as Apple ID. Yes, it could be a spare email, kept relatively secret. But that's just hassle, and no email address remains secret for ever. Easier for me to just disable FaceTime.
But to each his own.