How do you deal with it?

[Jakabella]

Bella used to do this all the time too. We purchased a wide black band that buttons around wone strap and then around the other and its to wide to let her get her arms out - its such an awesome little gadget! And it only cost us $20 - wel well worth it!

We bought ours from Oakies - but im pretty sure the would be at any big baby shop.

[Shanti]

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.