First, generally, judgments accrue interest, so you may have may have paid the amount "listed" on the judgment, but if payments have been going on for several years, it is possible a balance may still be due.
However, if you feel you have paid the judgment; fist send a letter to the law firm handling the garnishment and demand an accounting. Odds are, they won't provide one, but you need to at least ask. Then, you would need to go back to the court that issued the garnished and file a motion to cease the garnishment and declare the judgment satisfied. At least by getting it in front of the court, it will force the creditor firm to respond (or if they don't), the judge might just rule in your favor.
File a motion with the court to enter an order of satisfaction if you have paid the judgment. Until the court enters an order of satisfaction, the judgment will remain on your credit as an existing obligation and the court will continue to issue garnishments.
Jeffrey S. Kemm is licensed to practice law in the State of Michigan and the State of Missouri. This answer does not create an attorney-client relationship; you should exercise caution in disclosing information concerning your legal issues until you are certain you are protected by the confidentiality an attorney-client relationship creates. For additional information, please visit my website at www.kemmlaw.com.