Answer by Frank Heikens for Created user can access all databases in PostgreSQL without any grants

PUBLIC has access to the database by default, but it can't access the data. You can REVOKE the PUBLIC:

REVOKE CONNECT ON DATABASE your_database FROM PUBLIC;

If you want this setting for all future databases, revoke CONNECT on the template1 database (default template database for creating a new database):

REVOKE CONNECT ON DATABASE template1 FROM PUBLIC;