Postgres Server access restricted to only one database from list of databases
If you make changes to pg_hba.conf, you need to reload PostgreSQL. 

But I think you'd be better off revoking the CONNECT privilege for that user. (Or possibly revoking ALL privileges.) Skeleton syntax is 

REVOKE [ GRANT OPTION FOR ]
    { { CREATE | CONNECT | TEMPORARY | TEMP } [, ...] | ALL [ PRIVILEGES ] }
    ON DATABASE database_name [, ...]
    FROM { [ GROUP ] role_name | PUBLIC } [, ...]
    [ CASCADE | RESTRICT ]


Something like

REVOKE CONNECT ON DATABASE database-name FROM fishy_user;


In PostgreSQL, each role (user or group) can be granted permission to access different databases if that's what you want to do. See Database Roles