I believe this forum would benefit greatly from the reCaptcha plugin that ships with phpBB 3.2.
Is this running 3.2? That's the latest version, right? I think this forum is on an older version. Not all plugins are backward compatible. I've been visiting the forums since 2011, when 3.0 was the current release (it was released in 2007, and 3.1 was released in 2014). I don't recall any major updates to the forum, although I could've missed it. So, my guess is 3.0 (HTML page source doesn't indicate that I can tell).
But, yeah, something needs to be done.