You won’t find a lot of decent multiplayer tutorials on the web at large unfortunately, a few of us have even complained in the past that Epic’s official MP tutorials don’t cover the best practices. It’s a complicated topic and the best documentation is the source code itself, along with Epic’s ShooterGame example project - which is extremely comprehensive.
The GameSession is just an actor spawned by the Server - I figured my comment would be enough for you to go looking and find what you need. As has been said you can get the spawned GameSession from there. You will most likely need to subclass it however and add your own code, because the default engine implementation is pretty barebones at best. Again, ShooterGame is the place to look for info.
IIRC there is no “Ban” functionality implemented by default, it just routes straight to kick. That’s something you have to add yourself since the concept of banning is usually unique and bespoke for every game.