if I can be of assistance...
I'm running a phpBB board at my school, and have for some time been the target of porn ad attacks from Russia. Because I am too illiterate to install any security measures (like tricky jumping passwords on grey background a machine can't read - I saw that on some forums) I have taken to checking my memberlist almost daily, and removing any newcomers that don't look like humans (to my skillful eye the newcomer today is a robot who will post a 'penis enlargement' ad one day). While doing this, I manually remove any members who haven't posted within the last month. For them it's not a problem, as they can register again under the same name.
Here you've got about 270 non-posting members which can be a problem... But if you remove, say, 5 a day there's a chance to get rid of them before the end of the century.

edit: added the bit about today's newcomer