NetMQ close connection of peer using ROUTER socket

Question

I am trying to implement a load balancing message broker using NetMQ and the Paranoid Pirate Pattern. The client communicates with the message broker via TCP over the Internet, which is why I want to implement a transport encryption and an authentication mechanism for the clients. Transport encryption seems to be possible using curve but I have no Idea how to do an authentication mechanism properly. The thing I am unsure about is how to close the connection of a client that failed the authentication.

The authentication flow should be something like this:

1. Cliends sends a "HELLO" message to the load balancer. This message contains a token
2. Server received the "HELLO" message, reads the token and validates it.
3. If the token is valid, the load balancer responds with its own "HELLO" message, indicating to the client that he is properly authenticated
4. If the token is invalid, the load balancer closes the connection to this client.

I don't know how to do step 4. I know that there is something called ZeroMQ Authentication Protocol (ZAP) for that but I don't know whether that is already implemented in NetMQ and if so, how to use it.

tl;dr How to close the connection to a particular DEALER Socket connected to a ROUTER Socket in NetMQ?