Maybe adding an additional table where teams have to put their robot when it is out of bounds (and not touch it) could do it?
Of course if the team plans to repair the robot they have to turn it down, but otherwise, it should stay on that table.
This way the out of bounds robot cannot send any information, except the fact that it is not playing, it looks less suspicious and it is harder to cheat.
I personally think it is really hard to control the robot that is in the field since it is playing in a very dynamic environment and robots are much faster and more precise in decision making than humans, but of course, someone may get creative.