I was recently overpaid in the form of a performance bonus on a check a month ago. They now want to garnish my wages by $400 a month which will crush me. I used the money to pay off debt, and honestly thought it was not a mistake. Do I have any legal right in this matter?

Honest mistake or no, you owe the money back. Have you tried to work out a repayment agreement? And just FYI, they are not "garnishing" your wages. A garnishment is a court order requiring your employer to withold from your net pay to pay another creditor (not them).

Do you have any legal rights here? Basically, no. It is truly amazing how many people post here who claim that they didn't know they were being overpaid, who think that just because it was the company who made the error that they should either be relieved of repayment altogether, or think payments should be "spread out" to accomodate them.