Now, looks like many spammers love to use proxy to put ads in people's sites under the comments section. By using proxy, shows that these don't mind revisiting the same site eventhough the site is aware of their spamming and banning them. As long as they get to put the ad, it would look like they've done their task and very likely are getting paid for that.
Since proxy servers mostly use a kind of protocol, the majority of such usage can be blocked using simple coding. In .htaccess or php. I opted for php because it is much simpler:
<?PHP IF(ISSET($_SERVER['HTTP_X_FORWARDED_FOR']) || ($_SERVER['HTTP_USER_AGENT']=='') || ($_SERVER['HTTP_VIA']!='')){ DIE("Proxy servers not allowed."); } $proxy_headers = ARRAY( 'HTTP_VIA', 'HTTP_X_FORWARDED_FOR', 'HTTP_FORWARDED_FOR', 'HTTP_X_FORWARDED', 'HTTP_FORWARDED', 'HTTP_CLIENT_IP', 'HTTP_FORWARDED_FOR_IP', 'VIA', 'X_FORWARDED_FOR', 'FORWARDED_FOR', 'X_FORWARDED', 'FORWARDED', 'CLIENT_IP', 'FORWARDED_FOR_IP', 'HTTP_PROXY_CONNECTION' ); FOREACH($proxy_headers AS $x){ IF (ISSET($_SERVER[$x])) DIE("You are using a proxy."); EXIT; } ?>
After this, I made a few modification, to have a counter for each of the 2 cases. This is most efficient against those who use proxy in their own browsers.
As for the rest, I may have to disable url in the comments in the future, because I do keep a spammer free site.
These spammers are really restless, in spite of the Captcha implementation.