collections/harassing

Nov 2010 i pd in full and closed an acct i had. they claimed i did not pay, so i wrote a ck instead of doing on-line. they still stated i did not pay. each statement i got (8 mo worth!) had a different balance on it... i have made numerous calls to first source and HSBC (another co they are associated with who also send statements which by the way, don't have the same balance as the other one!). i left detailed msg, acct # ect and have not once received a call back. this has been 8 months of calls. the last call i made (today 6-3-11) i told them to stop harassing me, left my acct # and name. what can i do about this mess? they don't return calls, who can i contact! Mgr