How can I customise the WD My World Book Edition II users' permissions?

I want to make the guest account be able to copy stuff BUT not delete or add. I have the device set up on my home network filled with content that I'm sharing with my family. They're always accidentally deleting stuff or cut-and-pasting and moving around the folders. In the admin page, when I play with the guest account permissions, it always ends up having two results: 1. They are able to copy, add, and delete. 2. They can't see the content. There must be a technical solution for my annoying family members.