The first time that “NoMark” tried to knock out a Flock camera feed, he waited an hour for the perfect moment – pacing in and out of bushes that towered over him, a few feet behind a pole holding up the device.“I was just so nervous,” he said.NoMark, as he’s known on Instagram, had already gained an online following of hundreds of thousands for posting videos of himself wearing a black mask and taking on small-time vigilante missions: breaking up fights outside a bar and breaking into a refinery...