There's a few options for doing this.
1) On the client that you want muted, on their publisher object you call publisher.publishAudio(false). This will make it so that nobody can hear that persons audio. Documentation on the publisher here: http://www.tokbox.com/opentok/api/tools ... isher.html
2) Alternatively, instead of cutting off the audio at the source (the publisher) you can cut off the audio on the receiving end (the subscriber). In that case, if you don't anybody to hear someone, call subscriber.subscribeToAudio(false) on the subscriber that you do not want to hear. Documentation on the subscriber here: http://www.tokbox.com/opentok/api/tools ... riber.html
What you could do, is have the person you want muted send a signal() to everybody, then look up the connection id on the signal on all the other clients, find the subscriber for that connection, then call subscribeToAudio(false) on that subscriber.