Many thanks to Gary (aka "VALENTINE") for his discovery of a very effective <$20 throttle-lock. Some riders may have an issue with it slipping.
For $1.00 @ ACE HARDWARE you buy a small piece of 1 inch "non-skid" adhesive-back tape (generally applied to ladder steps --it has a coarse sandpaper texture to it).
Slit it lengthwise into a 1/2 inch X approx 5 inch strip & apply this to your grip. The throttle-lock now fits over the tape.
Result: No more slipping & still easy to disengage.
motorcycle throttle control