Well, this is a trick someone gave to kill those spammers a run around.

RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} .wp-comments-post\.php*
RewriteCond %{HTTP_REFERER} !.*azrin.net.* [OR]
RewriteCond %{HTTP_REFERER} !.*athira.net.* [OR]
RewriteCond %{HTTP_REFERER} !.*marlinda.net.* [OR]
RewriteCond %{HTTP_REFERER} !.*visitkent.eu.* [OR]
RewriteCond %{HTTP_REFERER} !.*citizennews.eu.* [OR]
RewriteCond %{HTTP_REFERER} !.*marlindaradzi.com.* [OR]
RewriteCond %{HTTP_REFERER} !.*fixingmyfixation.com.* [OR]
RewriteCond %{HTTP_REFERER} !.*islamictimes.org.uk.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^$
RewriteRule (.*) ^http://chat.nu/?Channel=Go-Fuck-Your-Self-Spammers$ [R=301,L]

Change the HTTP_REFERER to your domain names, so you don’t have this error
Save it as part of your .htaccess

If you still don’t understand, tell me.