Not a good idea. How will you determine how often to send the space?
Well it could just be as simple as checking to see if any key is pressed or not. If a key (wasd) is pressed then send it out, otherwise send out something that isn't one of the keys the code is looking for. He can code it to just send out one stop key or a continuous stream. He could even press 'x' and send it out to stop the motors.
Its up to him as to what he wants to do. Also he didn't say what he was using to send the chars in the first place, so he could already be sending out a NULL char and just not be looking for it. Who knows. 