UPDATE: This article has been updated as of May 12, 2020 to reflect the current and accurate information regarding stimulus checks sent to deceased individuals. Below is information directly from the IRS website.
A Payment made to someone who died before receipt of the Payment should be returned to the IRS by following the instructions about repayments. Return the entire Payment unless the Payment was made to joint filers and one spouse had not died before receipt of the Payment, in which case, you only need to return the portion of the Payment made on account of the decedent. This amount will be $1,200 unless adjusted gross income exceeded $150,000.
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 economic impact payment.
For Paper Checks - Indiana
Kansas City Refund Inquiry Unit 333 W Pershing Rd Mail Stop 6800, N-2 Kansas City, MO 64108