I've had that before. I copy and paste the user id to my blocked list. But you need to go to the end of the list, place a comma (,) after the last name, then a space and paste the user id. It sometimes says not added so I just do it a second time and it usually accepts it the second time.
I don't know why it does it but more often than not I have to try twice to get it added. If they are really bad buyers I also add their email address. But I don't even know if you can find that now - not had to do it for ages.