Anti-Spam Pro plugin is an extended version of the free Anti-Spam WordPress plugin. The plugin blocks spam in comments. It blocks spam silently without any captcha or math questions. It is easy to use and it starts working immediately after activation and does not require any setup.
Anti-Spam Pro blocks almost 100% of automatic spam by tricking the bots to fill in the input fields which real users would never fill.
Anti-Spam Pro can also block manual spam. with inbuilt settings to customize what words are to be treated as spam, and the limit for the comment etc.
Both versions are rate high about 4.8/5 for Anti-Spam and 5.0/5 for Anti-Spam Pro.
You can try free Anti-Spam WordPress plugin and see how it works for you before trying extended Anti-Spam Pro plugin.