Banning players from chat should disallow them from talking in raids as well.
Banned viewers can use raids as an opportunity to do the things that got them banned from said channel.
Another idea to fix the suggestion I have is if someone gets banned, they will not be allowed to watch the stream either, and they cannot access the twitch channel page for said streamer.
A simple fix in Twitch's code would be to not allow banned, and maybe timed out account, to join the raid.
People right now can manually not participate in the raid, and it seems simple to leave the banned account out of the raid.
A little extra code could make it that banned account can't even see the post-raid host details. The account would be left behind on the offline channel.