The plugin worked for a few days. Then I'm getting complaints from various readers (I don't require registration to read the blog), that they are blocked.
Sure enough Spam Blocker is blocking all readers, not just spammers.
Deactivating the plugin does not fix the problem. I'm not sure if it is a cookie stored on the person's machine that incorrectly flags a valid person as a spammer or not.
This is a serious bug that needs to be addressed.