Well, I have a question. What if you DON'T want your players to act like this? Just.... tell them to stop, punish them in the game or just say "screw this" and stop playing?
Just talk to 'em. The group's got to be on the same page about more than house rules and game system; the agreed-upon mood of the game, the reason for playing, the notion of how seriously to take it, to attempt for drama instead of comedy...
talk to 'em. If they're on board with your desire for a more serious game, just ask them to change the tone and play it straight. If they're all having fun playing around and being goofy, look into a light-hearted game that encourages/allows that...if you're on board, too.
It all comes down to a group consensus, with this sort of thing. You don't want half of the group showing up for
Grand Theft Auto's bleak shades-of-grey morality and angst, and the rest of the group showing up to play
Saints Row The Third, with it's crazy over-the-top hijinks and open amorality; they're both games that can fit pretty well into Shadowrun (for instance), but the two campaigns would have very different feels, right? Sometimes you've just got to talk to your players, and find out what mood folks are in.