There're already apps for that https://play.google.com/store/apps/details?id=com.sg.sledog

Every time after a user gets a spam call and hang up, he can tag the phone number and the tag will be stored in the database and used for all the other users.

So the more people tag, the more they benefit each other. If the call on your screen shows that it was tagged as freud, you don't have to answer it.