If the trailer is attached to the tow vehicle, it can help to back it up to an immovable flat object, like a fence or the wall of a building. Once they get it disconnected, it's harder to move.
I had a friend in my RC flying club had one of his utility trailers stolen even with Usual padlock through the latch and a lock on the tongue and very distinctive trailer markings with airplane decals etc.
For exactly this reason, DH has never put model airplane markings on his trailer, and only a very small scale modeler builder sticker on the Odyssey. Markings for ID should say something like "Joe's Plumbing."