Here’s a cool blog post that will help you fix the problem. I already applied that patch and it works great!)
It contains some modifications you have to apply to your box2d(2.1a).
This patch fixes a bug with contacts pull and contacts factory that caused memory leaks. Plus it clears the b2Body internals for quicker garbage collection.
Also, you can force garbage collection using System.gc() from time to time; But a call to this stops debugger execution; at least it did so on Adobe Flash CS6, so beware)
Once again, thanks to devizgl for his fix! )