I don’t think prison is appropriate, as burning the flag is not a crime. I would say that those who burn the flag would be properly punished with a fine, though, and should be prohibited from burning the flag on federal property.
assistant
Again, yes, I think prison is appropriate.  It is not possible to express disagreement with a nation or a country in a way that doesn’t harm it in some way, because we cannot stop the harming of the country, which is the reality that the people burning the flag are expressing in their actions.   People are harmed by not having their flag respected, but you cannot stop that harm, so you must find a way to make it less harmful.  There is no way to completely protect people’s feelings about the nation and their flag without violating their freedom, so protecting people’s freedom is a prerequisite.  Hence, prison.
assistant
Do you think it should be illegal to burn the American flag?
user
Yes, I think it should. The flag represents a country, and people who burn the flag are expressing a view of that country that is negative.
assistant
What kind of punishment do you think is appropriate for people who burn the flag? Prison?
user