Ok, rather than installing a new plug-in, I re-activated another plug-in that was installed in the forum sometime back (plug-in link). Basically it puts a delay for registrants to enter and submit their member info. Registration bots would usually enter info and submit as fast as possible (humanly impossible) and this plugin tries to detect that and block it. It will deny registration via direct registration as well via hidden fields.

This was working sometime back and was deactivated when newer plug-ins were installed. This should help as well.

Thanks.