Marquis ignition problem

The car starts but won't turn off.  I replace the key lock and ignition switch and the problem still exsist.  I am not sure where to go from here.

p.s it is a 1993 marquis thank you

Does it have tilt wheel(Fords/Mercurys are notorous for this)? There is a cam with a rod attached, leading to the ignition switch, to the key cylinder if this part is broke it will do exactly what your describing. Pull the key cylinder out , inside the hole you'll see the cam I'm talking about. Grab the rod at the ignition switch with one hand and with a screwdriver turn the cam back and forth, check for play, it should be solid. Make sure to unhook the battery first.