1986 Ford F-150 Ignition doesn't disengage

I have had the starter replaced three times because sometimes when I turn the key, the starter stays running after the truck is running. The starter kills the battery and shuts off. Last time I was lucky enough to make it to my local mechanic. They replaced the starter again even though I told them it seems the power to the starter doesn't turn off even when I try to turn the truck off with the key.

Help!