You can keep a reference to the joint when you create it like this:
var myJointDef = new b2RevoluteJointDef();
revoluteBall_jointDef.bodyA = ...
... etc
myJoint = world.CreateJoint(jointDef);
The 'myJoint' variable should be global, eg. you could declare it here:
var myJoint = null;
$(document).ready(function () {
var b2Vec2 = Box2D.Common.Math.b2Vec2
,b2AABB = Box2D.Collision.b2AABB
... etc
Then you should be able to destroy the joint from any point in your code later.