Mix up a glob of JB Weld and then screw the cap in. Once that sets, it is never coming out. The other answer would be to get a rubber connector boot bigger than the pip and connect a new piece of PVC which you could cap.
vbd
Purchase a No-hub clamp at a plumbing supply house, along with a cap, or cleanout plug. The No hub clamp slips over the damaged threads, and connects the new fitting. Tighten the screws. Done!