In any publicly-accessible forum, spam and trolling are a huge issue, consuming most administrator-hours and requiring a larger staff than would otherwise be required to run a large site. In comparison, sites that charge for admission, and sites that cannot be joined without an invitation have close to no administration overhead beyond maintaining the code, servers, and resetting forgotten passwords.

Some sites have implemented reputation—so users can take responses with the appropriate amount of skepticism. I suggest another, less transparent reputation value.

Pair the “report” function of a site with another reputation, one that is a ratio of posts reported to problem users identified. For users that have a ratio of 1 (100% of posts reported were by problem users that have been banned/warned/whatever your protocol) and a minimum number of reports, allow their reports to take effect without approval, with random spot-checking as the administrators have bandwidth. This would be implemented as a review queue (rather than the report queue), and could be browsed with less urgency than live issues, and would maintain high quality results.

This sort of merit-based promotion of volunteer moderators could be automated and invisible to non-administrators, the blessed moderators not knowing their own abilities, just that the site is a better place for their effort.