"Block the user"--that requires the app to have a block feature. Would you say that's something to be taken for granted? Something we can rightfully expect in communication software? Something the service-provider has an obligation to provide?

If 'bullying' in a social app/service continues, then yes, there is option to stop using it.

But if that service has value, users are going to want to keep using it, but they're going to ask for a better service.