Simple: Make it inconvenient. Require anybody who marks a post inappropriate to fill in a textbox explaining themselves.

Apply a basic 'stupid filter' to submissions (just to strip out submissions which are gibberish or vulgar) and make it clear to people that you will be filtering on those things.

Set a reasonable limit - 250 characters or something, so editors don't have to read essays on why a submission is inappropriate.

A little inconvenience goes a long way in shutting down site abusers. ;)