On May 6, 2020, the IRS came out with the following guidance for Economic Stimulus checks sent to dead people. The whole answer is at www.irs.gov/coronavirus/economic-impact-payment-information-center#more (See question 41)
In summary,
A Payment made to someone who died before receipt of the Payment should be returned to the IRS in full. For payments made to joint filers and one spouse had not died before receipt of the Payment, 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.
If the payment was a paper check:
If the payment was a paper check and you have cashed it, or if the payment was a direct deposit: