Doesn't that come out to an average of 5000 posts per user? Which would be far above a sane minimum for cheating.

That not only eliminates much of the filtering (although the distribution of posts per user might still be uneven enough to filter out some users), but it also brings the (non-filtered) number of hash entries up to anywhere between 5e6 (an even 5 posts per thread, and everyone is cheating) to 2.5e10 (an even 5 posts per thread, but no user pair is repeated) to 1e12 (some threads are sufficiently large, and everyone meets everyone else).

You could reduce the memory requirements a little by running it in two passes. One to count votes, and the second to count coincident posts only for user pairs that voted for eachother enough times to be suspicious.