It can be checked through lawsuit -- 8chan could conceivably sue Cloudflare under numerous doctrines, starting with breach of promise / breach of contract.
Another service provider could step up, as was the case with Daily Stormer, and is extensively commented upon in Prince's commentary, and provide services.
Regulatory or legal procedures could be established to specifically address this situation or provide redress.
Public outcry, market sanctions, or labour actions might be taken against providers who exercise such power in manners which are seen as morally reprehensible. For similar examples, see Google employees over Dragonfly or Edleman's emplyee backlash over a contract with a border-wall services company.
The question to be asked, the question we all have to ask, is whether or not individuals, groups, companies, or inchoate movements which are themselves dedicated to abolition or denial of civil order and rule of law are themselves deserving of its full protections in pursuing those ends. And a considerable case can be made for "no".