I have tried adding the banned email domain to Options->Banned Email Domain, but this one still cannot prevent the same spammer from registering.
I have installed Anti-Splog, but only use reCaptcha, however, it seems that this may still be an automated registration.
Is it due to direct access to the registration page wp-signup.php? But when I tried to access this page via browser, it's redirected to the main blog (I am running BuddyPress). Or is it a bug for banned email domain for WP 3.0?