While there are some pretty great Bluetooth headsets out there, even noise cancellation can't replace a real live mute button sometimes. Hacker Jared opened up his headset and put in a switch of his own, with just a little re-wiring.
Jared's particular headset, a Scala Rider FM, had two parts, one of which was a larger receiver that held the noise cancellation features. It might be difficult to do something like this on just a tiny dongle, but if you have a fancier headset with a receiver it should be pretty simple. You'll just need a little experience with electronics.
All Jared did was de-solder one of the mic connections and add a second wire to the solder pad, placing a switch in between the two. Thus, when he flipped the mini-toggle switch, the mic would turn off and no one would be able to hear any noise from his end. You'll probably need to re-seal it with some silicone, too, to keep out rain and such. Hit the link to read more, and share your favourite Bluetooth hacks in the comments.