Gah, it's so annoying when people trade you, you cancel, and they keep at it for 5-6 times. They should get the hint after 1 cancel. It's harassment :P Here's my suggestion..
If they attempt to trade you after 2 cancels, they can't attempt attempt another try for 5 minutes. If you want to trade them, you can establish the trade instead.
Either that, or strike them down with a fat lightnigh bolt at 3rd trade attempt :P
L

