The bulk of the spam you’ll get using phpBB is generated by scripts, so there’s very little time spent actually "creating" it. Because of the way Google works, if someone can get a link to their site on a whole lot of other sites, their position in the Google listing will rise. So they have spiders that search for various sites using popular scripts, and then send them generated messages or create accounts the same way. I have the same problem at a phpBB board I used to have. There are some plugins you can get that require the person to put in a number displayed on a graphic before they can create an account, and even to post if you want to use it for that. That’ll cut out the bogus "members".

I’ve avoided the problem here because the entire site I made myself, so there’s no reference point for them to spider.