Check out
NorthernTool. They sell a lock set (Item# 125580), from DeadBolt
that includes a hitch lock and a trailer pin lock. The pin lock fits PERFECTLY in the spare tire mount hole. The lock twists in from behind while you hold the key and provides a snug fit. Between this lock and the locking lug, I doubt it will be easy to jack the spare.
Here's a couple of pics:
