The Stovebolt Site Forums The Parking Lot Big Bolts Key off, starter motor continues to run

It's possible, but not very likely given the 100A break rating on the relay that the contacts are welding together and can't open. Breaking a circuit is harder on contact points than closing, thus the big difference in rating on the relay. But again, the solenoid coil on the starter would draw way less than 100 amps. It's easy enough to test by monitoring the voltage at the coil terminal on the starter solenoid. If it stays hot when you turn the key off, then the relay is bad.
Like Jerry said, the starter solenoid itself may be gummed up.