Analogy for a university listserv gone bad (click through for the whole very funny piece):
The rest of the faculty are crowded together down on the field, wearing football helmets and running into each other at random, over and over and over again. There are no referees.
I actually run a listserv for a small professional society where any subscriber can post, but so far we’ve been safe. The default Reply setting is To Sender, not To List, which definitely helps. Also, I try to keep a pretty high bar on the things we send out—either member involvement, or 100% match with the mission, or an opportunity for publishing or funding. No possibly related calls for conference proposals.
But I suspect I’ve just been lucky.