Discourse, which is pretty much Stack's successor, already solves this problem:
Your level of permission is capped: there is no "karma"/"rep" number that users are motivated to grow forever. There is no automatically-granted permission level beyond TL3, and all TL3 users are equal.
If you dip below the activity requirements, you lose TL3 (you can't lose TL2, but TL2 is pretty easy to get).
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
Discourse, which is pretty much Stack's successor, already solves this problem: