I would think you could make a case for denying free expression to illegal aliens. You could probably make a case for denying free expression to incarcerated criminals.
Other than that I cannot think of anyone else one could deny free expression to.
Therefore, If there is a public square, I do have the right to stand there and speak aloud on any subject.
If there is a public space set aside for announcements, then they have a policy whereby decisions are made as to what to include as an announcement in that public space. They are required to administer that policy impartially for those who request use of that public space.