Spammers
I see you're still getting spammers registering and posting despite the word verification. At another forum I'm on, they implemented a simple question on signup that only locals (US and maybe just PNW) would know that has put a complete stop to it. This may be custom work though and not part of phpbb. I'm not sure if you're willing to dig into the source code. If so, is there some question that could be added that is obvious to those interested in the housing market, but external people wouldn't know? Something like "What does NAR stand for?" or even asking what the name of the blog for this forum is? Something to break the automated spambots.