From the often funny but always legally interesting Popehat:
American employers are, in fact, responsible for taking reasonable steps to protect their employees from racial or sexual harassment by third parties. This is the example I use when I train companies on sexual harassment prevention: if the UPS guy is constantly and creepily hitting on your receptionist, you need to do something about it. You may think that it is outrageous that this is the rule. Cool story, bro. That’s what the law is, and if you employ people or advise anyone who employs people, you’re a fool to ignore it.
If you are running moderation services, you should read the whole thing and then maybe take a lunch with your friendly corporate lawyer to make sure you don’t end up on the wrong side of this.
At the very least, take it seriously if one of your employees starts complaining.