Making it a knife-free zone won't help at all, because criminals will not obey a "No Knifes" law any more than they obey a "No Guns" law.
Oh! You meant that by making it a Knife-Free Zone the brain dead will think that they are now protected. Maybe if we gave each one of them a kevlar No Knifes sign that they could hold up in front of somebody they might be safer. But would they carry it all the time, or just when they thougyht they might be attacked?