What is the easiest way to block a certain email address from sending you messages in Gnus?
The easiest way without using external programs or writing dozens lines of elisp code is to use splitting.
Let's say your spammer is named John Smith after the well-known Time Lord, and you store your mail in a group called
mail.misc. Just put this in load path:
(setq nnmail-split-methods '(("mail.spam" "^From:.*John Smith.*") ("mail.misc" "")))
The second string inside the alist item is a regular expression.