How to stop spammers posting spam comments to your wordpress blog using plugins to filter the spams.
There are lots of ways to avoid spam comments in your blog, but i only will give you a tip that i personally have used myself (since I refuse to use lots of plugins).
Using some plugins, captcha is alternative of combatting spam comments in your blog.
If you want to get rid of spam bots, read this post about how i finally can get rid of spam bots in WordPress.
These are what i have done, to prevent spam comment postings in my blog (using wordpress 3):
1. Installing and activate Akismet plugin.
One of the best wordpress plugin that is very important to prevent and combat comment spams. Find more how to activate akismet using Akismet API key here.
2. Choosing that you, as admin, will always moderate the comments before they are displayed in your blog.
3. Filter spammy words, IP address, email address in “Comment blacklist” option.
Be careful banning IP address because it is possible you will ban the whole city or even nation.
Both no 2 and 3 above are more explained in discussion settings wordpress tips here.
4. Choosing a nofollow blog to prevent following a “spammy link” is a good idea if you can’t really decide which links are spammy and which are not. WordPress is by default a nofollow blog.
5. Using wordpress spam plugins, such as wp spam free or capctha might help too. I personally would take this step after all 4 steps above do not work as using too many plugins often cause problems.
More resources from wordpress.org: http://codex.wordpress.org/Combating_Comment_Spam