I would do this:
1. IMMEDIATELY dispute the $300 charge if it has been processed. Tell the bank you did not authorize it. THEN CLOSE THE CARD AND HAVE THE BANK ISSUE YOU A NEW ONE. That should get you the $300 back. If the $300 hasn't been processed, CANCEL the card NOW!
2. Send the company a DV letter. Make them prove that you owe the money. In your OP you said that all your PDL's were PIF, and you closed your old ckg account on April 11, 2006 - but, he said $ was deposited on May 23, 2006. Hmmm...makes me wonder...(scam here)? Maybe he had the date wrong, maybe he meant 2005? If so, and you said your PDL's were PIF then there should be nothing to collect...
3. Chances are they are an illegal PDL. I would file a complaint with the AG, BBB and FTC. The fact that they first misrepresented who they were and threatened you with a felony, THEY are the ones breaking the law.
BTW: If you send the DV letter and they don't validate, then they cannot collect. If they continue to try, you could sue them for illegal collection activity.
Also, spend some time on this website and see how others have handled similar issues.
Hope this helps!