[Rac3r]

OnPlayerKeyStateChange, check if the player presses KEY_FIRE, check if he's in range of the zone, if so TogglePlayerControllable(playerid, true), or as Rachael suggested SetPlayerArmedWeapon(playerid, 0).

Using OnPlayerUpdate, you can imagine how many times SetPlayerArmedWeapon(playerid, 0) would be called, way too much.