How to prevent connections using default RAC database service

In some cases, it is desirable to have control about user sessions, which is why services are used. For example, when doing a switchover operation in a Dataguard environment, new connections will automatically connect to the new primary if they connect through the proper service and if it is configured correctly.

But how can you enforce the use of configurable services? Since the connection definition is at the client side, you have no control about it.

The only way I can think to achive this is to prevent connections using the default service (the one that is automatically created), so clients are forced to use the service assigned to their applications.