If the tension is correct, just snug the lock nut down. You don't have to tighten it too much, as this could cause other problems. Always be sure to loosen the lock nut before adjusting the screw. Older Lyon and Healy harps will have back plate screws without locking nuts. They have a flat shoulder which sits flat against the action plate when tightened.
Tension is adjusted by adding washers inside the screw.
Salvi back plate screws are different. Their actions use Teflon washers on the disc side of the action plate. as well as on the back plate. Also, the spindle is threaded into the center screw, instead of sitting against a spring and a washer. Because of this design, Salvi back plate screws can be tightened, without harming the action, until any buzzing disappears. You will also notice that the discs won't push in toward the action plate.