they can't keep their records straight

help...I had gone to a school that has been unable to keep their records straight about $400 since 1999. I recvd an apology for their error of sending this to a collection agency,that they notified the agency and that their records were updated to reflect that I was paid up in full letter in JUNE 1999 .
Now Dec 2000 I recvd another letter stating I owe $400 again spoke to a woman faxed her the letter.She stated she would update their records,apologized stating that since they keep moving locations all their records are not correct and this WOULD NOT go any further.

Jan 2001 recvd a Collection letter from a new agencyon the same $400...

What are my rights against these people who may possibly damage my good credit? I am in NEW YORK

HOW CAN I GET THIS TO STOP?

I feel that they are responsible for getting this taken care of already. I have been and keep doing my part ..good thing I retained all my info from them otherwise I'd have to pay this $400 .. I am getting very stressed out and feel as I'm being harassed and getting promises that they never keep...