In order to take it off your credit report, you have to file a dispute to the credit reporting agencies and state that it is not your debt.
If it *is* your debt, then it will be on the credit report for 7 years.
Inquire with your HR Dept at work as to how the garnishment is taking place (ask for a copy of the court order) and then call the court and file a Motion to Dismiss due to improper venue (you have to prove that they filed with court case without your knowledge because you were served the summons in an improper manner - i.e. you never got it).