From a self defence viewpoint, that would be regarded as pretty much spot-on, the weakest part of a grip is the thumb, and what you effectively did was push the thumb against the bottom of the bar, breaking the grip. Simultaneously you gave him a loud and clear verbal instruction - that's basically what we teach somebody to do if they have been grabbed (for example) by the wrist. (Also standard self defence teaching is act straight away, not allow somebody to get "fixed" into a situation).
I notice that you're flying the Streak. That's just appeared in the UK (in the XT912/StreakIII guise), how are you finding it? I flew a couple of hours in one, and found it pretty benign and very well built, but unnecessarily heavy in roll (especially trimmed fast, I guess due to the lufflines being fixed at the kingpost instead of running through a pulley like most UK wings) with what seemed a very low Vne compared to what it would do in level flight.
G