From the Economic Impact Payment Information Center on the IRS website
Q41. What should I do to return an Economic Impact Payment (EIP)?
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.
Do not 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.
For your paper check, here are the IRS mailing addresses to use based on the state: