What you're experiencing is neither preignition or detonation, but dieseling or run-on. There is a hot spot in the chambers, could be the edge of a head gasket, a sharp corner on a piston top or even the spark plug electrodes if you are running extended-tip plugs. Whatever is hot is acting as a glow plug and still allowing the motor to run because of the air-fuel mix that is getting past the throttle blades at the idle setting.

Install an electric solenoid on the throttle arm that only allows the blades to come to the idle position when the solenoid is energized with 12V from the key. Otherwise the blades are completely closed and the motor cannot run on because it has no air/fuel to combust.

Or you can do it the hard way, disassemble the motor and polish everything to within an inch of its life