It's not that easy, honestly. You don't just "stop the payment" and walk away.
It's called a "voluntary repossession" and carries the same penalties as a regular repossession. Your credit's ruined, and you're still liable to pay the difference between what you owe and what the car brings at auction (so be prepared to cough up thousands). If you can't pay, they can get a judgment against you and start garnishing wages.
If those are going for $15,000 at auction right now, you could be looking at owing $25,000 or more AFTER you turn it back in.
I'd do anything you can to avoid it. Call your lender and work something out immediately. You're another victim of the SUV market bottoming-out due to rising gas prices. Also, if you put down $24,000 at the time of purchase, this tells me that something has happened in your life if you cannot afford gas in the car. You must also realize that if you turn this car back in, you've flushed that $24K down the toilet.
If you're looking for a "magic" way out of this, it doesn't exist. So many folks take the advice given here and scoff at it, but there is no other answer.