Edit in JSFiddle

var A = _.extend({name:'A'}, Backbone.Events);
var B = _.extend({name:'B'}, Backbone.Events);

var whosListeningToMe = function(){console.log(this.name);};
var whosListeningToMeAgain = function(){console.log(this.name);};

B.listenTo(A, 'whosListeningToMe', whosListeningToMe);
B.listenTo(A, 'whosListeningToMe', whosListeningToMeAgain);

A.trigger('whosListeningToMe'); //logs B and B

//tell B to stop listening to specific event and callback on A
B.stopListening(A,'whosListeningToMe',whosListeningToMeAgain); 

A.trigger('whosListeningToMe'); //logs B