I have an idea that would keep users from "bumping" up their own threads. How about if the thread starter posts more than once consecutively, the consecutive posts will not move the thread up. For instance:

Spetnik: This is the thread. <-- Moves to top
Parksie: My member stopped throbbing <-- Moves to top
Spetnik: Yeah, I noticed it's vibrating <-- Moves to top
Spetnik: Btw, why'd u change ur avatar? <-- Does not move to top
Barrk: Because I stole the cute rubber ducky. I needed to take a bath <-- Moves to top
Spetnik: Finally. <-- Moves to top