I think the appropriateness of the punishment depends on the activity being punished. If this was merely a protest (i.e. a non-disruptive display showing opposition) then I don't think any punishment is appropriate.
That said, if she was actually impairing the speaker's ability to speak or the listener's ability to hear, then I think some punishment would be appropriate. I don't like the idea of making an adult write an essay as a punishment (especially if the essay expects the person to espouse an idea that she may not agree with). Compelling a person to recite the underlying rationales of the First Amendment seems inherently contradictory.
Assuming some punishment was appropriate I think I would've preferred just the community service.