Overall, I've gotta say I'm a little disappointed with the responses I got. Maybe I should have said Arthur Petrelli instead of Hiro, so I could get some replies that didn't hinge on "Well, Hiro's a pretty nice guy, so..." :p
First, let's look at some initial constraints. Hiro can teleport, so any solution that involves putting the jewel in some hard-to-reach place is kind of wtf. You can also assume that Hiro could easily watch you as you're hiding the jewel wherever. He can't become invisible, but he can move fast enough that he can't be seen. Any solution that depends on him being a pretty nice guy is kind of suspect, too, since he might not always be so nice. Why, in five years or so, he might end up being a badass, and carry a sword and stuff. :o
Traps are also mostly useless, since Hiro can pretty much just stop time and avoid all of them. However, traps that would destroy the jewel could potentially be useful. If Hiro causes the jewel to be destroyed, he won't be able to "get his hands on it", so we may assume that he won't do that. Care must be taken, though - if he can stop time and get the jewel out before it's destroyed, then he's won.
The neat thing about using traps, though, is that we can use them to force Hiro to stop time. If we can then find some kind of mechanism that requires him to leave time moving to get past it, we've caused a contradiction: Hiro can stop time or not, but he can't do both simultaneously.
So here's the setup: a safe with a bomb attached to it. The safe is small enough and strong enough that any attempt to cut through it would destroy the jewel. Furthermore, surrounding the safe on all sides are lasers; if any laser is interrupted or disabled, the bomb will go off. However, the lasers can be disabled, with a password known only to you. The safe has a combination lock, but it's fragile: If you try to open it too quickly, it will break, and blow up the safe. Everything has backup batteries, so you can't just cut the power; if the baterries get too low, the bomb will go off.
I actually left out the requirement that you have to be able to retrieve the jewel in the initial problem, so I've actually solved a somewhat harder problem here. Ah, well. In any case, there's my solution.