Those who deliberately harass can be brought to the attention of whoever operates the service they're using. If what they're doing is bad enough they can be warned, kicked off the system and, in some cases, prosecuted. Check with your service provider to find out its policy. Here are some guidelines I came up with a few years ago for keeping this new medium civil. Know your recipient.
Different people have different ideas of what is acceptable.
Find out and respect each person's wishes. Avoid sarcasm, unless you're sure it will work, and think very carefully before using e-mail to express anger. With e-mail, once it's sent, it's gone. If you're posting a message in a public forum, remember it can be read by a wide variety of people. Be careful about your use of irony or even some forms of humor, which can be misinterpreted.