There is no banned user group in XF, but you can ban members of a given user group in a batch.
In the ACP, go to the users tab and use Batch Update Users. Select the user group you want to ban, then click search. Then check the 'Ban Users' box and click 'Update Users'
Banned is banned, they have no access to the site.
It sounds more like you should create a user group that can only access the one forum where disputes are filed and move the users to that group without totally banning them.