Nothing that can't be defeated given access and time with the right tool(s).
Do you have a fence and a gate? That is first.
Is it a bumper pull trailer or GN?
You can get a GPS tracker for cheap and mag mount it under the deck (wood, not metal). The device will notify you via text message if the trailer moves. Then just call the cops and have them go get it and the perps too.
A good insurance policy.
i put a chain through the wheels. i know someone could drag it, but if they are that dedicated, i am not sure they would get too far down the road before they had an issue.