Hammering! You gotta be kidding me. As my old boss used to complain "You need a license in Pennsylvania to cut hair but ANYONE can mess with a car braking system." Sad, but true.
Remove the dust cap, crack the bleeder screw, and push the piston back into the bore by hand. For one thing you don't want moisture laden brake fluid that is in the caliper bore to get pushed back into the brake lines. For another, on a good caliper the piston should slide back into the bore with hand/finger pressure. If it doesn't you should pop the piston and clean out the bore/piston and install new seals. A piston that requires clamping to retract will not behave properly during normal use and will cause the pads to drag. Fix it right.
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