How can I toggle a astable 555?
The easiest solution is to simply use an AND (or OR) gate with one input from the '555 output and one from your ENABLE signal. 

If you try switch the 555's power on and off for each transmitted symbol, you could have a couple of problems: 1. Switching could be slow because the power bypass capacitors have to charge up and down as you switch. 2. As the power ramps up and down the 555 might produce pulses with wrong duty cycle or wrong frequency. 

If you want to save power, you could power the whole transmitter subsystem on and off on a message-by-message basis.