Runs after shut off

In a gasoline engine run on or "dieselling" can occur due to high engine idle, high engine temperature, deposit build up in cylinders. This problem usually occurs more often in the warmer weather when engines are operating at higher temperatures but if the conditions are right it can occur even in cold weather. Although the key is turned off and there is no ignition supplied to the engine anymore the engine internal temperature heat is high enough to cause the car to act like a diesel engine which runs by heat and high compression. A couple of things you can try for the time being is let the engine come to an idle for a minute before turning it off. You can also try shutting down the engine while in the drive position as the transmission will tend to act as a brake and slow the engine down. When the engine stops then shift into park position and then remove the key.
This usually works best. If your car tends to jump ahead when you put into gear your idle may be too high. Cylinder deposits can glow like coal internally and ignite the residual fuel much like a chimney fire where the deposits in a dirty fireplace flue ignite and burn. If the deposits in the cylinders are substantial the compression ratio can actually be increased and can cause engine dieseling.
Years ago you would be in the shop getting a carbon and valve job.Cylinder deposits today are less than years ago due to improved gasoline product and better oils, improved ignitions and fuel injection and computer monitoring of the system.
The problem of high engine temperature can be caused by different things such a dirty or clogged catalytic converter or exhaust, rusty cooling system, water pump impeller worn, clogged radiator just to name a few. You should have your car looked at to determine what is wrong.