Assuming you have mod_rewrite and AllowOverride activated, throw this in the .htaccess file for the directory you don't want to be Slashdotted:

RewriteEngine On
RewriteCond %{HTTP_REFERER} ^.*slashdot.*$
RewriteRule .* - [F]

This all assumes the editors actually check the links before posting the story, but this should reasonably mean no-one on Slashdot can link to you, even if it's a bit over the top.