Visitors are able to edit documents in a Board
Visitors are able to edit documents in a Board.
Normally, users with visitor status in a Board can't edit any documents.
This problem might be a consequence that someone has changed permissions of the Site in SharePoint that is laying behind the board.
The solution to this issue is to delete or close the Board where a visitor is able to edit documents and then, create a new Board instead. In this case, please remember to make a copy of all this Board's files and upload them to a new Board.
This solution will work only for Boards that exist now. When you create any new Boards, you should not experience this problem anymore.
What is more, also changing permissions in particular documents can cause this problem. If you break the permission inheritance on Documents they are not under control of KanBo anymore.