[mreid]

Do you have a mini starter or standard? If mini, how is it wired (firewall solenoid in addition to starter mounted)?

Starter run on can be caused by a sticky solenoid, dual solenoid conflict, bad starter, bad ignition switch, or wiring issues. Post more info on your set up. Also, did the engine keep running because it was being turned by the starter or was it actually running? If it was actually running and you had turned it off, this is almost definitely the ignition switch.