I reported a spammer with the plugin which worked (reporting successful), and saw the spammer posted again the next day. There were enough reports for the particular spammer IP on stopforumspam so it should have met the threshold. I don't allow visitors to leave a URL in the website field, does this plugin work if the email address or IP the spammer used is in the spam db?
I did test the internal blacklist and that worked fine. The plugin is enabled as it has the status "Checking with Stop Forum Spam and Project Honey Pot".