Joking: Pepper spray. When someone looks behind the screen when your find is back there, spray until the can is empty.
Serious: Ask them why they keep doing that, and if slowing down game
and lessening challenges are things they're really looking for in a game.
I once heard somebody on a different board outline his group's issue. They were playing 3.X, and somebody went online and found a bunch of "tricks" to "win" all the time. They'd fight, extend a Rope Trick (opens a pocket dimension where they can rest unharassed), and then they'd recover spells and kill another group. Then repeat. They did this about four times before the DM did something I thought was amazing.
When the group said "we exit the Rope Trick, and go through the door looking for the next group," the DM simply said, "you kill them, and are able to Rope Trick again. It works all throughout the rest of the fortress. You go on to level up, defeating hundreds and then thousands of opponents and propel yourselves to epic levels. You win."
They were dumbfounded. The DM said, "is this what you want out of the game? To win? Or do you want to play and take some risks?" The group decided they wanted to play, and they picked up coming out of the Rope Trick. They didn't use that "trick to win" or any others like it anymore.
Just make them really successful when they cheat. Look at an NPC or monster stats so they can kill it easier? The NPC or monster dies, then and there, for no reason (still gives full experience). Same things for bypassing traps. But, don't let the game progress like this, just skip over all the juicy combat, saying they win whenever anyone cheats. Then, like the other DM, ask if that's really how they want to play.
If that's really what they'd prefer (winning, but never getting to play out anything again), then you have an incompatible group (from the sounds of it). If it's not, maybe they'll stop, or the other players will make the 24 year old stop acting out.
Just my thoughts. As always, play what you like