A stripped screw on a cabinet knob makes removal seem impossible. The knob may turn while the screw remains completely frozen inside. Typically, a hacksaw is used to cut off the old screw head but that isn't possible if the screw head sits flush to the cabinet door. If just the screw head is stripped, it's sometimes still possible to remove the screw from inside the knob. If the screw has become stuck inside the knob, removal requires more effort and you may not be able to salvage the knob afterward.
Insert a screwdriver into the screw head. Tap the end of the screwdriver softly with a hammer so it sinks into the stripped screw slightly.
Grasp the knob tightly with a pair of pliers. Turn the screwdriver while holding the knob so it doesn't turn.
Select a drill bit slightly smaller than the screw head if the screw is completely frozen and won't turn with the screwdriver. Use a bit made for drilling into metal.
Drill into the center of the screw slowly until the screw head breaks off. Pull the knob from the cabinet door.