You Wrote:

So what do you do when you've gone back 15 edits, reverted to a clean version, incorporated all of the "good" changes that have happened since then, and the original perpetrator then comes along and adds his/her "bad" changes again? How long are you supposed to keep doing that?

Well, until you get tired of it, I guess. You're not obligated to do everything, Zoe. :-)

kq