SitePoint has a wonderful thread titled "10 Tips for Fighting Forum Spam" - I strongly suggest you read it. http://www.sitepoint.com/forums/showthread.php?t=427488
There's half the problem right there. phpBB and spammers are like milk and cookies - they literally go hand in hand with one another. I suggest you switch over to another forum package (one that's far more secure than phpBB). SMF would be a good candidate for this, especially since it has a package that can transfer your phpBB databse over so you don't lose any accounts or posts.