Throttle lock like above or a http://crampbuster.com/
Motorcycles - Page 315
Sounds like you're gripping too tight. Last winter I got a lot of cramps because I was clenching my heated grips for dear life. Relax the hands
My hands still get a bit sore around the thumbs after riding for several hours, but that seems understandable to me. Moving my thumbs on top of the grips for a while solves that issue.