If you have the -DE(Drive Enable) option on the motor.. you could inhibit motion by removing drive power.
You could also use the over travel switch inputs to inhibit motion(if you don't have the -DE option).
You could probably do the same with soft limits if you can send the commands via RS232. SLN=0 SLP=0 SLE
One more... you could set EL=0 and AMPS=0 so any motion command would fault out without moving.
...with any of these options.. you would have to set values back to what they were... and clear errors with a ZS to allow motion again.
The serial port has high priority... so commands like S can come through. I don't know of a way to ignore or block the motion commands without blocking everything.