If you have self hosted WordPress and always got lots of spam comment, you may actually blocked them through your .htaccess file.
The source code as below was get from AllGuru.Net,
RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} .wp-comments-post\.php*
RewriteCond %{HTTP_REFERER} !.*xyz.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^$
RewriteRule (.*) ^http://%{REMOTE_ADDR}/$ [R=301,L]
Open the .htaccess file and place the code as above then you shall be able to filter the spam automatically.