I would be careful about this. Could definitely be used for bullying by kids - would suggest you come up with a strong mechanism/policy for banning users!
You could say the same for a lamppost (that I can attach a photo with a message to) and just about every other means of communication where the sender is not known (regular mail, for instance).
Technology can be used for good and for bad, this tech is no different in that respect.
The lamp post is part of a wider local 'ecosystem' that includes ways of dealing with abusive messages attached to publicly-visible objects.
Communication apps like this, not so much.
This is why service and software makers are considered to have more of a responsibility to be concerned with misuse of their platforms. (ETA:I mean platforms, as in publicly-accessible stuff on the net, that's under their control.)
"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.
