Is it possible to block the access of a super user?
That should be easy. Put the databases in different PostgreSQL clusters on the same server.
But even if both databases are in the same cluster, a superuser can only access the database if there is a line in pg_hba.conf that allows it. So you can achieve the goal with a properly configured pg_hba.conf file. That requires that the superuser is known by name – you cannot add a line that says “block all superusers from that database”.