Gsdfsdfsdfgsdf kdsfiezrzer > CHEAPGILZBUYNOWZ!
Gsdfsdfsdfgsdf kdsfiezrzer > CHEAPGILZBUYNOWZ!
Gsdfsdfsdfgsdf kdsfiezrzer > CHEAPGILZBUYNOWZ!

[Right click name, select Send Tell]
/tell Gsdfsdfsdfgsdf kdsfiezrzer

[Replace /tell with /blist add, and add quotation marks around their name]
/blist add "Gsdfsdfsdfgsdf kdsfiezrzer"
[Hit enter]

Popup: "Are you sure you want to add Gsdfsdfsdfgsdf kdsfiezrzer to your black list?"
[Click Yes]

Annoying? Yes.
Should there be a better process? Yes.
Does it work? Yes.
Are you getting any more spam from Gsdfsdfsdfgsdf kdsfiezrzer? No.