Windham Professionals / say I owe them money that have already been paid by IRS

Windham professionals has call many times after I already paid all of my loans by withheld taxes from the dept. of Fl. Educ. They still continue to call after I have paid the balance. Isn't this against the law? They say I still owe them? I have paid my student loan off twice over I am sure. The lady said they IRS will never pay it in full, "thats how they get more money from you". She even ask why I had the money to pay for my son's glasses before I paid off my bill. Give me a break. What can I do about this? Can they garnish my wages like they say, If I do not owe?