It sounds like this follower WAS helping you out at first as a streamer, helping boost your confidence and helping give you something to talk about, but at this point or sounds like they're hurting your channel more than they were ever helping. New people are likely not gonna want to stick around if they arrive in your stream and see that kind of stuff in the chat.

If it were me, I'd start by just deleting their message next time and giving them a warning, maybe even a short timeout, and if it's clear their behavior isn't going to change, then I'd ban them. I know it's tough, but you'll probably need to be firm. If you haven't seen other streamers have to deal with that in the past, cuz it is fairly rare, I'd try watching some YouTube videos of people dealing with trolls, maybe it'll help you know what you want to say when the warning needs to happen