"Those who know they can manipulate the participants to trash the thread are the most guilty of the mess that happens."

So make your job simpler. When they start, yellow card. One more strike and you're out. There are approximately four individuals who foment trouble. I'll name names if you challenge me to. You got rid of two others and the place improved immeasurably. You got rid of another who wasn't doing much harm but hey ho. One problem I've gleaned is that two of the worst offenders have garnered the sympathies of a mod each. Doesn't exactly help, does it?