From GM to DM (that's what the DND people call the Game Master right?), punish the destructive players. Whether it be by making it so they gain no experience for what they've done because of their behavior, kicking them out for a session or two, or refusing to run until they've calmed down, whatever it takes. You're the one with the authority and you're the one doing it for them.
No offence, but you sound like you're not taking a stance and letting them walk over you. Sometimes you have to get their attention and respect by not tending to them, but making them realize your power.
Now I realize I may sound a bit cruel and drunk on power right now, but the fact of the matter is that you've tried being nice to them, and it's not cutting it. Sometimes people have to learn the hard way.
You've offered them the carrot and they refused, now give them the stick.
Oh, and on a final note, make sure you give respect to them and the game as well. If the game is very lax in general and you're doing stupid stuff, you're just going to have a bunch of idiots dicking around. Not to say that isn't fun though, hell I've run some of the funniest games by just throwing the rules out and going for it, but if you want them to act serious, you have to make the environment serious.
No offence, but you sound like you're not taking a stance and letting them walk over you. Sometimes you have to get their attention and respect by not tending to them, but making them realize your power.
Now I realize I may sound a bit cruel and drunk on power right now, but the fact of the matter is that you've tried being nice to them, and it's not cutting it. Sometimes people have to learn the hard way.
You've offered them the carrot and they refused, now give them the stick.
Oh, and on a final note, make sure you give respect to them and the game as well. If the game is very lax in general and you're doing stupid stuff, you're just going to have a bunch of idiots dicking around. Not to say that isn't fun though, hell I've run some of the funniest games by just throwing the rules out and going for it, but if you want them to act serious, you have to make the environment serious.