Unplug it and find out.
Beware, the plug may be for the blower motor and may not disable the heating elements.
If the heating elements run without the blower, the unit will overheat, it may cause a fire or burn the unit up (electrically).
Best bet is to find the line feed to the unit that you want to disable and properly terminate the connections there. Proper termination consists of capping the line feed wires with wire nuts and enclosing the end of the circuit in a metal box enclosure.
As an alternative, you may want to determine how the unit you wish to disable is activated by the thermostat and disconnect the wires that control the unit. This the unit will not come on with the other unit but, can be easily reversed to restore operation.