The times when DD has gotten out of the straps (put her arm out) or pressed the release button and said "uh oh" () I have immediately pulled over, looked her straight in the eyes and said "we do not take our seat belts off in the car, for your own safety" firmly but gently and this seems to have worked both times. She hasn't done it for ages since.
If she ever was to do it again I think I'd look into one of those things you can buy that you're all talking about but I'm lucky she hasn't done it for a while.