Can a creditor garnish your income tax

Yes, if he has a court award.

Can be garnisheed by only the order of a court of competent jurisdiction, and that too depends upon your state.

Once the person gets the state court award, he/she can file in FEDERAL court to garnish your federal tax return.

It's a detailed and drawn-out procedure, and it will not work once the taxpayer realizes what is happening and stops overpaying his taxes, but it CAN be done.