That depends on whether or not they cut the ignition wires and spliced it in there.
On my Spec project car they cut the ignition lead and the alarm had a relay to control it. If they did that on your car you simply need to patch the wire back.
It's not too difficult to uninstall an alarm. Just unplug the wiring harness from the control unit and trace the wires back to find where they were patched in and repair as necessary.
RJ