The best solution I've found to stop WordPress comment spam is the 'Text Captcha' plugin.

How to install the Text Captcha plugin:

1. Log into WordPress.
2. Click on the 'Plugins' button on the left.
3. Click on 'Add New' under 'Plugins'.
4. In the search field enter 'Text Captcha' then click the 'Search Plugins' button.
5. Click the 'Install Now' link for the 'Text Captcha' plugin.
6. Click 'Activate Plugin'.

Your done... Now a captcha problem must be solved every time someone tries to leave a comment, preventing bots from spamming your site.

Note: If you want to edit the questions asked by your 'Text Captcha' plugin, click on the 'Plugins' button again on the left side of the screen so the drop down options appear then click 'Text CAPTCHA Admin'.

When you're through making changes click the 'Update Settings' button.