I don't think they need to ban the likes of Katie Hopkins or Qanon, as long as they break no laws impinging on a person's privacy and hate speech. Hopkins was mostly careful to phrase her tweets in a way that came out as personal opinion rather than race hate. I may not agree with you but I defend your right. blah blah.
People are quite capable of knowing the difference between fake news, CT and opinion. For example there is now one account claiming Meghan Markle is a part of a cannibal elite as well as being a 'former yacht girl', introduced by former client Prince Andrew and that Prince Harry has a double and is not really with her. The clue is in the account avatar of a frog. (Frog emoji= alt right) Does anyone really believe this? Of course not.
Sure twitter should ban accounts that 'violate rules' (spam, porn, hate speech, persoanl abuse) but I can't see how it can police opinion and speculation, no matter how outrageous.