|I suspect the most common coutermeasure will be handing out Vicodin (the most powerful C3 pain releiver that is also widely available in the US). Not really that effective but doable.
Costs $2/dose (unsure of US street price)
Now for effective, I expect people to make an aluminum/mylar foil exterior bunny suit (like in some Chip FABs) As I understand it , the Zapper frequencies are in the microwave range, so a thin metal coating should block the effect on the human nervous system. The question is how much shielding is needed. Too much and you got bulky slow moving protestors.
Costs: estimate $150/person
One other posible counter measure is a portable EMP burster (one shot weapon). Supposedly this is now small enough to be treated as a Heavy Weapon ie. 3-4 people to manuever the EMP device within range and fire.
Costs about $15K and requires a high energy physics lab to produce and test.
Well back to the paying Info work.