Did you ever send anything certified mail, return receipt? This is what you will need. Without it, a judge will not know if you actually sent the documents. Now the cancelled checks should help your defense. If you can prove it was paid, you will win.
They can report this for 7 years on your credit reports. If it is reporting incorrectly and you have proof they verified it was correct after disputing it, requesting debt validation, etc, THEN you have a case for a countersuit. Do you have credit reports, certified mail return receipt letters, or any concrete proof? How are they reporting the tradeline? Can you prove to anyone (i.e. a judge) that you really disputed this and they verified it?