I've never seen a small engine that was intended to be stopped routinely by pulling the plug lead off the plug. Very old B&S engines had a metal lever attached to the cylinder head that was used to short the plug lead to earth when you wanted to stop the engine, but that was pretty crude: you sometimes got an electric shock when using it.
It is normal for the ignition system to have a kill-switch that shorts out the low-tension part of the magneto when the speed control is moved all the way to the low speed position. Honda mowers have this feature, and I'm guessing your slasher does too. If it isn't working, you may need to adjust the speed control cable.