this may have been suggested already, (i read the first 10 pages, then skipped to the last) but what about a system like the "trader rating" system we have set up now. have something like a "Player Quality" or "Player Reputation". it worked for our classifieds, weeded out all the flaky sellers/buyers.
maybe make it so only hosts could write down public comments, like "player was removed from game due to safety issues." or "player was repeatedly warned not to take head shots", etc.
just a idea...
Guardians of Asgaard