This is usually fairly easy to discourage. Set up your discussion list so that only list members can post to the list, and immediately remove any members suspected of spamming. Also, some mailing list programs allow you to set things up so that messages which are rejected by the list do not get bounced back to the sender. This makes it harder for spammers to figure out what's going on. Another potential problem with public discussion lists is the tendency of people to go off on tangents, and start generating lots of messages that have nothing to do with the list subject matter.
Vulgar language is an issue that often creates flurries of tangential messages.
Lucy Brown uses a colorful expletive in a message, Polly Peacham tells Ms. Brown to keep her offensive language to herself, then Jenny Diver chimes in, calling Ms. Peacham a prude and extolling the virtues of free speech. Other popular tangents are viruses, urban legends, conspiracy theories and other similar things. To say nothing of stories about people having their kidneys stolen, or being shot for flashing their lights at someone on the highway, etc. etc. At the risk of going off on a tangent myself, I'll recount an amusing anecdote. A member of a stock-investing list that I subscribe to had his car stolen by some members of another racial group.