Using any proper logic that driver would not have been there w/o the rally. He came from out of town for that event. 
No anti-protesters, no crowd for him to drive into. No anti-protesters, no riot for the media to cover. 
If you want to take a stand, take a lesson from Jesus, Ghandi, or King. Line the streets and as the parade marchs by, silently turn your back. No need for rocks, urine bombs, or pepper spray. 
If you want Free Speech to espouse your views, you have to grant similar rights to other view points - no matter how distasteful. You don't have to listen. You don't have to respond. You just have to let them speak w/o violence.