In childrearing, my philosophy was let them do anything they wanted, with the limitation that "only so long as it is not too much trouble to keep a watch that they don't kill themselves" (or someone else).
" Is there no line that can be crossed between holding an opinion and seeking harm to others?" I think the line is the infringement of the other's liberty.
From "The Rights of Man" 1789. Article IV "Liberty consists in the freedom to do everything which injures no one else; hence the exercise of the natural rights of each man has no limits except those which assure to the other members of the society the enjoyment of the same rights."
As for enforcement. I do not think there is such thing as "a right to privacy in public space".
To control violence I would have all public space under continuous (webcam) observation with facial recognition and AI watching for any developing situation.