You need image verification mods or new integration of Captcha (in new phpBB version or phpNUKE 8.1?) like in any elegant website ,when they need you to confirm if you are human...(think when you apply for mail in Yahoo ,google etc. you need to put alphabets that can be recognized easily by human but bots can not understand)
I have mod on my site if you don't mind. get if just 3 places to mod.
http://nuke.thaihealth.net/article206.html