Are you sure if they died in 2020 theyre not entitled to the credit?
I can see if someone died in 2018 or 2019 they wouldnt be entitled to a 2020 credit....but if they had income in 2020 and are filing a 2020 return, seems like they would be entitled just like anyone else in 2020.

From the Draft Instructions:

 

Generally, you are eligible to claim the recovery rebate credit if in 2020 you were a U.S. citizen or U.S. resident alien, weren't a dependent of another taxpayer, and have a valid social security number. This includes someone who died in 2020, if you are preparing a return for that person.

https://www.irs.gov/pub/irs-dft/i1040gi--dft.pdf#page=58

I agree with TGB, but if you want to know how to prevent a 2020 credit if you truly believe they don't deserve one-- enter the full amount as if they already got it.

Done.