First of all, the collector has to validate the debt. Look how to do this, and do it ASAP.
This post:
http://www.fairfaxunderground.com/forum/read/2/1595705.html
has some more good advice.
If you really think the debt is uncollectible per your bankruptcy, write the collectors and tell them to cease and desist.
If they do not cease and desist, sue them in Small Claims Court for harassment and inflicting mental distress. Just make sure you serve their registered agent, not the PO box they use. This information is easy to get.
If they don't show up you get a default judgement against them. Now they have a problem.
Somewhere I read some guy did this to a collection agency. He could not find their bank account but they did have a website for making payments. He attached their domain name and turned off the payment thing. They paid him real fast as he'd just nuked their cash flow!