Many people join that just create yahoo ID's to get on the list for creating havoc. What's the answer to this?

People want to create a disturbance on some lists. So they creat a new yahoo ID just to join your list and then cause trouble. How to avoid these people? It would be nice to check them out on yahoo profiles before allowing them to join but I haven't found the way. Usually I have to wait till after they join. Then I have to ban them when I find out they just created their profile.. I would love to know a way to make it easier to just prevent these kind of people from even joining if I knew they were joining for ill intentioned reasons.