Can I cap old, live wiring to remove the switch?
That's a good start, but you will still have live wires running around in your walls for no constructive purpose.  

Find the breaker that controls the circuit, and turn it off.  Check all your other installed loads (sump pump, etc.) to make sure everything else still works - the several heaters should be the only thing on the breaker, but sometimes people do odd things.  Once you're sure of that...

Turn off the main, take off the panel front (be warned that if your main breaker is inside the panel, some metal parts inside the panel are still energized!!)  Then pull the breaker off the panel if it unclips (as it's easier to work with this way), remove the wires from the breaker, cap the wires in the service panel, put the breaker back with a N/C label on it, and put the panel front back on.