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.