View chat when banned
It doesn't hurt anyone to just allow banned users to view the chat without the ability to write something.
Exactly right. Being allowed to view the chat will let them know what their bad conduct has caused them to lose. This can move them to give up trolling/spamming. Someone who stops spamming/trolling would be a force for good in anyone's stream, including the channel they were banned from, assuming the broadcaster/mods who banned him/her are the forgiving kind.