Dont' think so. I think you can only protect the letterhead itself (protecting the rights to open, modify, view, etc). Once they have it open, they can do what they want to it. Of course, that's why QC and Q/A was invented.
If you made the template read-only, no one would be able to change the template, but only if they opened the template itself. If you used the template in a document, you'd still be able to change the template's effects on the document.