In the case of a trailer, you do some combination of...

- Receiver pin lock similar to the one highlighted here (but probably not that exact one) - Wheel lock / boot - Receiver coupler lock (locks inside the cup-shaped receiver, preventing somebody towing the trailer with an undersized ball) - Secured storage lot / garage

But, basically all options are only going to stop random opportunistic thieves. If somebody really wants whatever you're protecting, they'll find a way. That's why insurance exists.