For example, I'm in favor of banning speech exhorting folks to violence against people of any specific race/religion/ethnicity/orientation/belief. That seems like a simple enough restriction.
But then, in the event of a law like that being passed, the advocates of violence switch their tactic to explaining why said target group is subhuman, or resort to apophasis, or develop coded language.
The solution often proposed is to consider purely verbal instigators of violence to be innocent of any crime, and consider the individuals who physically enact the violence as bearing all responsibility. Unfortunately, having personally witnessed mobs getting worked up and seeing firsthand the collapse of individual thought, I fear this approach is based on an over-generous analysis of the rationality (and individuation) of the human brain.
I don't have any solution to propose.