Reply To: private network with 2 gw

You could implement a blacklist using an application

In node-red use MQTT in and out nodes with a function node between.
This code will clear the join accept packet out of the downlink queue.

if (msg.topic.search("<BLACK-LISTED-EUI>")) {
    msg.topic = msg.topic.replace("joined", "clear");
    return msg;
} else {
    return null;
}