On my last two trailers, there was a loop welded on the stem for the safety chains - I just bought one of those cable locks that's adjustable to lock at any length, put an extra ball inside the coupler, latch it closed, run the cable through the latch and up through the loop and back, pull it snug and lock it. This locks the latch and doesn't allow the coupler to drop down far enough to remove it from the stem. It could be cut off with the right tool, I guess, but I figure it makes mine more trouble than 80% of the other trailers in the lots - either it works, or nobody wants my trailers anyway... :-\
I've also seen home-made ones with a piece of pipe big enough in diameter to cover the coupler, capped on the bottom, and a piece of chain welded to the top and locked to a short chain or loop welded to the trailer above it.
My current trailer doesn't have the same loop on the stem however, so I'm contemplating buying one of the coupler lock/ setscrew lock combo's EZTL50 / EZTL51 here;
http://www.redneck-trailer.com/2006/K/K12-K13.pdf