There isn't functionality in the ActiveSync protocol to allow for what you're talking about (limiting its ability to affect unread flags, delete items, etc). I suspect you're going to have a tough time finding what you're looking for.
You could do a "hack" by making a second mailbox for each phone user specified as an additional / alternate delivery recipient for their "primary" account and "point" the phone at that secondary mailbox. The advantage would be that changes they made in that "clone" mailbox wouldn't be reflected in their "primary" mailbox.
The disadvantage would be that changes they made in that "clone" mailbox wouldn't be reflected in their "primary" mailbox. Their "Sent Items" from the "clone" mailbox would be separate, as would be any other folders or items they create / file. It'd be a totally freestanding mailbox.