Most likely you could change the usergroup unregistered to something else
or you could search for Unregistered in the phrase search and have "Phrase Text and Phrase Variable Name" checked. Then press search and find "unregistered", click edit and change the text to "Visitor" or anything you'd like.
Then when guests post they will have something like this:
I'm sure if you'd like to make it so they can't edit their username you could go to the newreply.php and edit the box where the username text is and make it uneditable.