Many options:

- block access ala GFW, ensuring that most people will have difficulty accessing it or using it

- block access to any data you cannot decrypt or from an endpoint you cannot backdoor

- go after creators and ensure some kind of backdoor is inherent to the project

- shut down projects by exerting pressure on developers

- run some kind of propaganda campaign on the evils of using unsanctioned software (supporting terrorism etc.)

- do nothing, knowing that most users will avoid using anything that isn't one of the major web platforms