You can put an electric solenoid-type throttle stop on the carb and stop the run-on. Lots of smog engines had them. Turn off the key and the throttle closes completely, it can't run then!
Or just let it idle for a few seconds to cool off the chambers. Shutting it off in gear works too, but not the best solution. They can kick back, blowing gas out the carb with the attendant fire hazard.