[Wise Man]
Violence is not the answer, Grasshopper.
[/Wise Man]
A Total Party Kill, on the other hand, is a viable option. As are Ethereal Filchers stealing all their magical items while they sleep.
Perhaps if you explain the situation, we could give you some more specific advice.
I agree that you should start by talking it over with them in a civilized manner.
If all else fails, institute a rule that everything said is "In Character." That way, if a player cracks a joke or throws out a Simpsons quote in the middle of the game, the King or Tribal Leader that they were negotiating with at the time might be offended. You also might try giving XP penalties or other negative effects for disruptive behavior.
Just remember that OOC banter is, and always will be, part of the game. It shouldn't interfere with the game, however.