Block referrer spam from the SEMalt botnet

We are trialling the following solution for blocking what is fast becoming a scourge of the web; SEMalt referrer spam.
It’s been added to the .htaccess file on one of our personal sites, and we’ll see how it does there before implementing these measures on the rest of the sites we manage. Update to follow.

UPDATE: While blocking at the server IS working, there is much more to blocking this type of spam which is detailed here and here.