Ask them not to do it anymore. If diplomacy doesn't work I agree with Tricia, use your admin tools. Switching individuals who can't control themselves to moderated mode is very effective, if you have that option. If your software doesn't support individual moderation, you may have to suspend the subscriptions of unruly subscribers. If the whole list is completely out of control, consider a temporary switch to full moderation.
Diplomacy is usually the first choice.
If asking nicely doesn't work, don't be shy about using force to maintain order. The list is your responsibility. Do what's best for your list and your subscribers. If a few subscribers are persistently screwing up an otherwise good mailing list, you may have to get along with a few less subscribers. How far you go to salvage incorrigibles is up to you.