WP-Spamhaus is a plugin I put together a few months ago to take care of what I call, "the spammer got in the door" syndrome.
It checks EVERY IP that hits nearly every page of your WordPress blog to three lists:
1. a local whitelist
2. a local blacklist
3. the database of IPs maintained by spamhaus.org
An IP that match an IP in the whitelist are allowed access.
An IP that matches an IP listed in the local blacklist is unceremoniously banned.
An IP that matches a an IP listed at spamhaus.org is also banned however (assuming you take care of this) the user is provided a way to contact you via e-mail to to request whitelisting, AND they are told that their current IP is listed at spamhaus. You will also receive an e-mail notifying you that someone has been banned, along with the IP, so that you can investigate it further.
I've no problems with the current "recommended" WordPress plugins that battle spam, except that they work after the fact --
The spammer still gets to spam you. And they still get to suck up your bandwidth.
WP-Spamhaus will stop nearly all of that.
Details and downloads are here: