To mute audio on a Bluetooth headset, turn down the volume in the application that is providing audio.
For example, if you're listening to music in iTunes, drag the volume slider all the way down to mute the headphones.
You might still hear audio on a Bluetooth headset after:
The media keys on your Apple keyboard can be used to adjust the sound level of your Bluetooth headset, but not to completely mute it.
You might also notice that the Mute option is dimmed in Sound preferences.