1987 Chevrolet Suburban Dear Sirs Would Turn Ignition Off Starter Kept

Dear Sirs,
The problem with my suburban was I would turn the ignition off and the starter kept turning the motor over. Then I changed out the ignition lock and ignition switch. When I went to put the negative cable back on the battery, the starter still runs with the ignition lock in the lock position. I would appreciate any help you can give me at this point. Thank you. Robert Ball