Edit in JSFiddle

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

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

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

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

//tell B to stop listening to specific event on A
B.stopListening(A,'whosListeningToMe');
//logs nothing, B is not listening to whosListeningToMe on A
A.trigger('whosListeningToMe');