Or have the transmitter transmit nulls, but that's a waste of transmitter power and it doesn't take care of a turned off transmitter. And I've never heard of pulling down an rf receiver input (or up, for that matter).
I had the same problem when I did an rf control link years ago. It worked fine, until my transmitter turned off then all kinds of digits came pouring out. I used a simple code a mentor of mine had invented for a totally different type of project years before. Basically, my data is broken down into nibbles (4 bits). I transmit a preamble, which tells what version of coded data is being sent. Then, I put a '1' in the high nibble with the first nibble of data in the lower nibble, followed by '2x' '3x' etc. If I need more than 16 nibbles of data, I just repeat the '1x' '2x' etc data stream. My receiver is able to lock on the data stream and decode it with perfection, knowing it is receiving the correct data in the correct format, with a check sum at the end, so it even knows the data it received is correct. Then it knows the preamble isn't some fluke of random patterns. And with the preamble, different systems or parts of a network can ignore data not meant for it and take what is meant, and use the same frequency. Since this system was used to remotely open and close gates at a zoo, you certainly don't want to open the lion's gate when you meant to open the elephant's... etc!!!