My understanding has been that one of the main purposes of the UN is to attempt to prevent or deescalate wars, by providing a forum for diplomacy. To that end, the kinds of people liable to start wars are exactly the kind of people one would encourage to attend it. Now, it may not have exactly proven very good at that (though it's a bit hard to say when we can't look at what history since it's founding would have looked like without it), but still, kicking those guys out doesn't actually do anything about what they're doing, it just tells them that you don't like them, which they probably already knew.