[Toptick]

Sounds like a solenoid problem. I assume that when the starter was running the pinion was engaging the flywheel?

If it were the ingition switch, turning the key to off and removing it SHOULD negate the starter circuit.

If the solenoid is "sticky", the direct connection to the battery would eliminate any input from the key.

If this happens again, you ought to have a plan to disconnect the battery, or you'll burn out the starter. Replace the starter & you get a new solenoid.