I vaguely remember Malky (I think) commenting about the time it takes deleting all the spam, drug- and porn-vendor usernames that get setup on the forum, and this is a problem that until a few days ago took up an inordinate amount of my time on another forum.
That appears to be a thing of the past now though, since I installed 'The Humanizer'.
This is a modification to the standard PHPbb files that takes about five minutes to apply and inserts an additional question into the registration script that very effectively confuses the robot software that creates these dummy user accounts without inconveniencing real users.
Full details of the modification and download link care at this web page: http://www.phpbb.com/phpBB/viewtopic.php?t=383788