http://www.brahmalock.com/I bought one of these. Was $275 or so taxes in but IMO the device is SOLID and pretty slim chance a theif will try and breach on on a trailer parked in town in driveway. Expensive and more than I wanted to spend but I view it as piece of mind and a solid long term investment.
Guys at place where I bought it had good point on tongue locks. Yes some are tough to break and apparently the one on our quad trailer is considered pretty decent (California Immobilizer). However, if a theif really wants your trailer and what is on/in it they will just jimmy-rig something (chains) and tow your trailer away with the lock in place!
Seems the only way to make your trailer a truly undesirable target is with a wheel boot style lock.
Yeah could take the tires off as suggested abouve but this is not really a pragmatic solution if you use the trailer often, and if someone wanted your trailer IMO it would not be too hard to show up with a couple tires with the correct bolt pattern and a jack.