I pulled the plug lead off a Go-Kart I once had because it was stuck at full revs and can honestly say that it's not advisable, you can imagine the plug sparks once for every stroke of the piston and at full revs will be doing several thousand RPM, I was zapped god knows how many times in just a few seconds, luckily I was ok and would hate to imagine what it could have done if I'd had a dodgy heart or had been a child. personally I would try stalling the engine by either covering the exhaust with something other than a bare hand or applying the rear brake if it's in gear.