Ignition wont stop turning the starter

I have 65" Pontiac Catalina, sometimes the ingnition does not stop spinning the starter motor, I think it is because the switch is not springing back to run position.
If i turn the key back to off it stops but engine stop running too.
I am afriad of burning up the starter motor, what do you think could be wrong.
also is it just replace the tumbler or the whole ignition houseing as well

Replace the lock cylinder and the switch and be done with it.

Thanks Rod, does the entire housing need to be replaced,
I see ignition replacement switches, but not the housing, is the spring in the cylinder?

Sometimes the solenoid on the starter jams, or the switch solenoid/relay built onto it gets stuck. Sometimes the cable terminals are touching when they are installed on the solenoid. I hate it when people tighten without looking.

The spring is in the ignition switch.