Hmm....this actually doesn't sound like a bad idea, and I'm not even sure that if it were to be implemented that people would just start up new threads with the same topic, because if there's already one going with 1000 pages, a new (essentially duplicate) thread could be reported to the mods and nuked. 

Then again, most of us who like reading things on the forum avoid mega spam threads like those in question so I'm not sure how much of a problem they really are.  And although duplicate threads could be avoided, I've no doubt members in bounties/sig campaigns desperate to meet the maximum post count would find a way to do so somehow, and that might present a different problem that's hard to predict exactly.

Good suggestion, OP.  The only problem is that it seems like Theymos isn't listening to proposals from the community all that much.  We did get the "OP" tag, which is probably one of the least useful ones I'd seen in years, but aside from that there haven't been any new methods made available to combat shitposting in a long time.  Keep your fingers crossed, though.