Can I control a 240V motor using a SPST switch on one leg without causing damage to the motor if I leave the other leg hot?
Yes. it is done all the time.
Just keep in mind if you work on anything that there is still at least 120 V on the connections.
Disconnects open both poles for this reason.
To reinforce what BRICKEYE is saying, yes you can control the motor that way, but if you are using the switch as a REQUIRED DISCONNECT you need to remove all the hot legs.