It's instinctive, you won't be able to stop it unless you keep him inside or in an enclosure. More play to tire him out maybe, but if he's a hunter, it's not something you are going to stop.
Cats will hunt. Most cats can learn how to hunt even with their bells on. The only real way to protect wildlife from your cat is to keep them inside or only let them outside when supervised (even then it is possible).