You can create something for that in your game logic.
The peer receives a ban command from the host.
The peer then fires off an Ajax call to a dedicated page.
This page processes the request, catching the IP of the user calling the page.
This info is then processed and applied to the firewall rules.
Voila IP blocked on server
Another approach could be that if the player receives a bann messages, certain values get stored in the local storage preventing him or her to reconnect.
This could be undone by reinstalling the game or something, but could become tedious after several bans.