You might want to take a look at this:
Grip-Lock - Scooter and Motorcycle Security Lock
It clamps to your grip and keeps the brake applied.
They are easy to use, very visible (they come in colors...mine is bright green. It sort of looks like a big pickle.
) so a thief can see that there is a security clamp on the bike and hopefully move on to some easier pickings, and you definitely won't ride off with it attached.
Do a search and you'll find several different sellers.