I’ve been hearing a lot of people complaining about how much SPAM they are getting & WordPress being unable to stop them. Well I do sympathise for these people for being victimised by SPAMMERS but they are wrong in blaming WordPress. 
As far as I’ve seen, WordPress has one of the strongest SPAM prevention features out-of-box. 
These features are made more powerful by some excellent work by the contributors to WordPress by rolling out great plugins that go a long way in blocking SPAM. 
So here’s a small list which will help you in combating SPAM if you are a WordPress user.
Kitten’s Friendly Comments:-
Allows comments by e-mail addresses whose comments you’ve approved in past. The rest of the comments end up in moderation que. The development of this plugin as stand-alone has been discontinued as its now part of the WordPress 1.3 core but you can still get its v1.0 from that link for use in WP1.2 & WP1.2.1
Spammer Tar Pit:-
This plugin by Dougal, a WP Dev, is an excellent one for combating spam. Check it out.
WP Blacklist:-
This one works. Do check it out.
Shut Off Comments on Old Posts:-
If you want, shut off comments on posts after they cross a certain age limit & become adolescents.
CSPAM:-
Read it here, I myself don’t understand it.
3 Strikes:-
Much much better SPAM blocking, the concept is very good.
Kitten’s SPAM Words:-
This plugin adds a “Delete Comments as SPAM” button to mass moderation page of comments. When you delete a comment as SPAM, its eMail, URL, IP & links in comment body are all added to the SPAM Words List. So future comments matching that will be in moderation que.
Kitten’s Comment Pay:-
This is one of Kitty’s Good ideas. It matches the comment content against a list & if any matches are found, it presents a page which asks the commenter to pay for making a comment as his comment contain possible advertising.
Comment Killer Plugin:-
This is the extreme stuff, branded as “weapons grade plutonium” by Kitty. Be cautious while using this. If a comment is flagged for moderation, this plugin will delete it if its active. BEWARE.
Kitten’s Spaminator:-
Hot new plugin from Kitty, this is a combo of the Spammer Tar Pit & 3 Strikes plugins mentioned above. A must check out, this plugin improves on Marks original 3 Strikes.
Besides, you can also use Simon’s Blacklist which is quite extensive & is frequently updated.
Also, if you find that a SPAMMER is using same IPs to SPAM you, you can block those IPs through .htaccess file by putting in the code
[ini gutter=”no”]
deny from xxx.xxx.xxx.xxx
[/ini]
Here, the xxx.xxx.xxx.xxx is the IP of the SPAMMER.
But remember, you can use .htaccess file only if you are running Apache Webserver & if your host allows the use of .htaccess files. 
These should be of help to those WP users who don’t already have any of these weapons. You will find that you can give SPAMMERS a real hard time with these weapons in your arsenal.