Despite all my anti-spam plugins, I'm suddenly getting Register-bombed by some crap online gaming site, using names like "Online Adventure Games", "Online Sports Games" and "Puzzle Games". The emails address & IP are never the same (and look auto-generated, like "firstname.lastname@example.org"). The ONLY thing they have in common is the word "Gecko" included under Agent.
Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:184.108.40.206) Gecko/20090824 Firefox/3.5.3 GTB5
Is there a way to block spammers based upon their User-Agent?