but if you set the threshold high enough

That's one of those dangerous phrases that sounds good at first, but are many times are (perhaps unconsciously) justifications for detestable behavior. Just a note to be extra careful.

Say if someone's overall rep for a week is below -100

Not such a bad idea though. Instead, i would look at for rep that is x% below the average. And then, just make the person a candidate for banning.

that would require coding too, and discussion when a vote came up

If it's for excessive abusers, it would not need to be automated. A simple query would show weekly rep (for a reported abuser), and cause for a vote amongst the gods. They can put the monk on probation for 1 hour the first time, and double it from there. Probation could simply mean posts must be approved before being seen by new users.