You can also put domain names in the blacklist. e.g. they are spamming from different IPs but promoting the same website. In this instance you can still search the pending comments by domain name. Just like the IP search, screen refreshes, select all, spam can.
The other options are:
-ban an IP or IP range. As mentioned previously this could make it so legit people can't get to your site at all.
-put a captcha and annoy your legit readers that can't read the captcha and just leave.
-Those signup using (fill in name) are equally annoying as far as I'm concerned. I don't surf logged into anything and I'm not going to signup up for yet another service just to comment on your blog. You get more comments when someone does it spontaneously. Plus if the service is down that just adds to the PIA factor of leaving a comment.
-use a spam plugin and slow down your blog. There have been reports that Akismet uses crowd sourcing and spam cans some legit commenters (much like banning the IP via the .htaccess file). The GrowMap Anti-Spambot Plugin claims to be a better alternative but the additional plugin they recommend gave me false positives so I don't use either.
I really don't see these anti spam plugins being any less work than moderating the comments yourself. You still have to go through the spam screen (it the plugin dumps them there instead of just deleting them) and cruise the comments looking for legit comments that got designated as spam if you want to build your readership.