Keep in mind that both fish, snails, anc crustaceans make up a general part of most hawkfish's natural diets so it is not outside the realm of its natural behavior. Have you ensured that it is well fed? This may or may not help curb its aggression towards other natural prey.
I think the only way to get it out would be by utilizing some sort of fish trap, where the fish would enter for a piece of prepared food and not be able to get out. More info should be available with a search here or on a Google search for fish traps.