Well, we have had a pretty good run of peace and quiet but it appears that, just like jock-itch and athlete's foot, the cretins have found a way in.
Starting as of a couple minutes ago I turned on the option to suspend the "friendly" e-mail registration scheme and now all new users will have to be "approved" by an admin. I have been whacking about 15-20 obviously bogus users a day but it seems if I miss a day they get into mischief...