Is there a way to alter role for a specific database ? Eg: Alter role

Of course otherwise revoking connect from public would lock everybody except the owner and superusers out forever what you can do is limit the database's connection limit btw. which these days with usually just a single (application) user connecting is pretty close to what you are looking for.

With connection limit 0 it's about removing access from a database, i guess that is possible with

REVOKE connect ON DATABASE [dbname] FROM [role];