Is it possible to mark a page/card read only (even for Admins)?
No, everything is always permitted to admins unless (a) the card's content is managed in code or (b) the entire site is running in read-only mode, which is basically only used in cases of server maintenance.
You may want to consider having multiple administrative Role. One would be for routine administration, and several users could have this role. The other (the built-in admin role) could be more like root access on a server, and you would only occasionally sign in with an account in this role to do lower-level admin work (like editing these read-only cards).