First install Google messages and set it as default messaging app then long press the required message and block it, to make sure that you blocked the required sender you can go back to Samsung messages and set it back as default, go to messages > tap the three dots on the top right side > go to settings > blocked numbers and messages > block numbers, you will find the sender name there. I couldn't find any other solutions as of now since that Samsung messages can only black messages with numbers.
I found another solution, type the sender name anywhere then copy it, go to messages > settings > blocked messages and numbers > blocked numbers > past the name and add it, this will also work.
