To disable the autofire you'll have to disable autopilot automation first. At least in DBW that autopilot automation control is Toggle Level Autopilot in control settings.
BTW, when using the turret as gunpod in P-61B-15 you'll have to visit gunners position and disable autopilot there in the beginning of flight. Otherwise the gunner autopilot still aims the turret to other directions than forwards and you'll shoot somewhere else than you wanted with the turret guns. On the other hand, with this arrangement you can still jump to the gunner position, aim the turret and fire at someone attacking from behind, only with different trigger than normally as turret gunner. Aiming the turret back forwards again can be done automatically by toggling gunner autopilot on and off.