Our range of racks use castings that lock onto the towball. These are tightened by a large bolt that can be turned with a wheel spanner. To make it more difficult to steal, this bolt can be replaced with a special splined with a special key.
However the whole unit can still be stolen by removing the towball or even tow bracket.
A further option to stop items being un-bolted is to mess up the outstanding threads showing beyond the nuts. Think before doing this as it will also stop you unbolting them too.