One libertarian's view: These idiots have every right to express themselves, including buring the flag.However, those who hold views contrary to the idiots also have every right to express themselves, including preventing them from buring the flag.
If I, a libertarian, were present at a demonstration in which someone attempted to burn the U.S. flag, I would use excessive force to stop it and suffer, if necessary, the physical or legal consequences of beating the skulls of fools into gravy smears on the pavement.
How about this for a solution? From now on, everyone show up at a rally carrying a bag full of water balloons. If these jackals start to burn the flag, start throwing them. This would have the effect of putting out the fire, striking back at these ungrateful brats, and yet none of these children could whine about being set upon by right-wing bullies. Do you think cops or a judge is going to take a water balloon fight seriously? Just make sure you don't freeze the balloons before using them!