Lotus Notes clients can edit and save received emails. How do I restrict them so they are not able to edit and save received emails?
Sorry, but there is no way to do this in Lotus Notes -- not without custom programming, and not without breaking other Notes features.
The programming would be to apply an Authors field to all incoming messages and leave the recipient's named out of it. With that in place, you could change the ACL so that the user has only 'Author' permission (with the extra 'create documents' permission checked to make sure the user has the ability to send messages) instead of the more usual 'Editor' permission. But things would break because Notes updates hidden items in messages when users do things -- e.g, setting a flag for follow-up, and those updates would fail. I can't think of other features that would fail off the top of my head, but I'm sure there are more. The error message the user receives when things fail would probably be very confusing, too.
If what you really need to do is to make sure that a copy of unaltered original messages is kept, then you need to investigate the 'email journaling' feature of the Lotus Domino server. (Discussion of that would be more suitable on ServerFault rather than SuperUser.)