There are a few things that are good at getting odors out, so you can try the following:
1) Vanilla extract. Sounds odd, but take a paper tower and dip it in vanilla (or imitation), and rub it on every surface that smells. It eats up the odors, but doesn't leave any of it's own unlike cleaning products (don't use ammonia, bleach, or vinegar on wood). Let it dry in the sunlight if possible.
2) Baking soda. Kind of obvious, but put some new baking soda in a small open-top container and leave it in the closed humidor.
It might take a month or more of trial an error to fix this--and it might be that you can NEVER fix it. If it's just a 20-count, get a new one on cbid for $7. If it's bigger/nicer--I'd fall back on that "slap the kid and then make him pay for a new one" idea.