Do your best to be your best is the best I have come up with in my circling of this dilemma.
I'd be willing to bet that you hear that little voice everyone has but most ignore. I think I know what it's telling you, but it's just a guess. Whatever you do about this situation there are consequences, as with all situations really.
People lie and cheat and steal. But not all of them. There are some stand up folks out there. Be one of them, the stander uppers.
I wonder sometimes, standing in my puddle of imperfection, how people do the cheating and breaking rules they've agreed to abide by so often and easily, but then I remember that it's not my job to worry about them. Not my responsibility.
We have an anonymous 'we-tip' line for crimes out here where you can leave information about poaching. Maybe you guys have something like that out there or maybe a letter sent from a town you don't live in to the wardens in his area that they may want to keep an eye on this guy for such and such reasons.
Good luck figuring it out, though it sounds like you have, mostly.