what about a report function? users report bad comments, mods get the reports in a list and take whatever action. users that get their comments deleted receives a warning. two warnings and you're out. well maybe not that harsh, but you get the idea. i'm sure you have seen this system elsewhere
Well the logging function would be more then enough as long as every comment get checked by an op wich depend on the time it take to do this and the time the op have