If you feel a user is being inappropriate it's best to tell the Broadcaster - you can leave them a comment or write to them in their chat while they're broadcasting. Users cannot ban other users, so you would need to alert the broadcaster or moderator if there is one.
If you feel that your requests aren't getting noticed you can also email the username and issue to our content monitoring team at firstname.lastname@example.org
if you feel the users are violating our Terms of Service.
Most bans however will be temporary.