The controller will stop the motors when radio signal is lost. If this is not the case, it is very likey because of your receiver being \"smart\" and it outputing a default pulse when the transmitter is lost.
Diconnect the radio and the controller should stop motors and flash \"no ctrl\" on the LED. If you dont see the same with the radio connected and just the xmiter being off, then this means the radio is outputing something to the controller. The radio\'s manual will tell you how to change that.