Yes, a visitor can leave a comment on hubs, but only if the hub author has configured it to allow guests to leave comments without signing in.
I believe that the default is to allow anyone to leave a comment. You can configure the comment settings for all of your hubs by going into your "My Account" page, clicking on the "Comment" link and then going into the settings.
You can also configure the comment settings for each hub individually. If you wish to block comments from visitors who are not signed in, edit the COMMENT capsule in your hub and check the box that says "Only signed-in users may post comments."
I know that some Hubbers prefer to have only signed in users leave comments, but since I moderate comments, I allow anyone to leave a comment. If it's spam, I just delete it, and it never shows up.