There is nothing that will absolutely keep someone from stealing it if they really want to, best you can do is make it difficult as possible, or unlikely as possible.

I put mine in a surplus .50 caliber ammo can with hole(s) cut just large enough for flash/sensor/lens. Velcro holds cam in place. Long shank padlock (where lack slides off shank) locks the opening mechanisim and fairly hefty chain loops through hefty eybolts attached to can (and chain has lock). I have a couple pictures of it that I may put up sometime to give someone an idea of what it looks like. My first cam (homebrew) survived quite a few curious onlookers and a couple playful skirmishs from a bear without a problem.
Also could help if cam is placed far from the beaten path (maybe in thick stuff where people would be less likely to travel)