If you have accounts one thing to do is to set a (unknown to users) daily comment quota. Set it to like 20 or something you consider very high and make an alert

If you have accounts one thing to do is to set a (unknown to users) daily comment quota. Set it to like 20 or something you consider very high and make an alert fire if someone reaches the quota. At this point the user shouldn't be able to comment and you should review manually case by case.