https://www.irs.gov/coronavirus/economic-impact-payment-information-center#more
Click the link , question 41 at the bottom for addresses to mail payments back:
A41. You should return the payment as described below.
If the payment was a paper check:
- Write "Void" in the endorsement section on the back of the check.
- Mail the voided Treasury check immediately to the appropriate IRS location listed below.
- Don't staple, bend, or paper clip the check.
- Include a note stating the reason for returning the check.
If the payment was a paper check and you have cashed it, or if the payment was a direct deposit:
- Submit a personal check, money order, etc., immediately to the appropriate IRS location listed below.
- Write on the check/money order made payable to “U.S. Treasury” and write 2020EIP, and the taxpayer identification number (social security number, or individual taxpayer identification number) of the recipient of the check.
- Include a brief explanation of the reason for returning the EIP.
