My 315 poly keeps running after everything turned off..

It can only keep running if it’s still getting power to the ignition circuit. Disconnecting the battery doesn’t stop the alternator from putting out power.

Put a meter on the circuit and see where it’s still getting power from after the key is turned off. It could be a failed ignition switch (key) or perhaps it it connected to an “always on” circuit that keeps the ignition circuit energized.

If the ignition circuit is always hot, all you need to do is crank the engine and it will start and run until either the ignition circuit is interrupted or you remove the fuel.