Provide option to not add anonymous posters to user list

Issue Summary

Please add an option to listhandler settings to not add anonymous posters to the user list. For private sites where "only administrators can create new user accounts" it is unnecessary/not desireable for listhandler to create user accounts for anonymous posters. The "Block" setting is good as far as it goes, but what we really need is the ability to prevent the user account from being created in the first place.