Re: Lock for screw type coupler?
I seem to remember seeing a type of lock for those type couplers, it was sort of like a "C" clamp design that simply clamped down on top of the knob and reached underneath the coupler somehow. It didn't look too secure, just a little better than nothing. Like TD said, lock the safety chains, if not just carry a length of chain and lock the wheels, or the trailer to the hitch when you park.
My best two trailers have super duty safety chains welded to the tongue and I have an equally heavy pad lock that I lock them to the hitch with. Its not likely that anyone would be able to easily get though the chains, they'd probably have to cut the tongue or hitch off. (I also tack weld all the hitch hardware).