Netcode for Gameobjects (Netcode) provides a way to implement a connection approval delegate that can reject incoming connections based on custom logic.
Boss Room provides an example of such delegate
The code below shows an example of an over-capacity check that would prevent more than a certain pre-defined number of players from connecting.
SUIBJECT TO CHANGE:
In connection approval delegate Netcode does not support an ability to send anything more than a boolean back. Boss Room demonstrates a way to provide meaningful error code to the client by invoking a client RPC in the same channel that Netcode uses for its connection callback.