As soon as I am granted access to the WP plugin repository for my newest plugin called "MailChimp for WP" you can start using that plugin (while simultaneously running Newsletter Sign-Up) to take care of the comment form checkbox. The new plugin ignores spam comments. I will later implement this in Newsletter Sign-Up as well but haven't found the time for it yet.
In the future this new plugin will be expanded so that it can totally replace Newsletter Sign-Up for MailChimp users.
I expect the plugin to be downloadable in a few days, at most. I'll let you know! Thanks for your patience. : )