I'm not a huge fan of forcing information to a user's clipboard, but you do have that option.
Script trigger that fires when entering a field you want to protect. Dialog box asking if they wish to edit, copy, or cancel. If they click edit, and their permissions allow it(maybe a value list of authorized users to make edits), it lets them into the field. If they click copy, contents of the field are placed on the users clipboard, if they click cancel the script doesn't allow them into the field.