Best bet: educate people, so that if they're dangerously contaminated they *don't* run towards other people and get them killed unnecessarily too. Failing that, ask the fleeing people to stop. If they don't, then it comes down to forcibly defending the uncontaminated people from the contaminated people. I wouldn't be shocked if that took guns. Panic makes people do stupid shit. What was that Spock phrase? "The needs of the many outweigh the needs of the few" or whatever. If killing 100 contaminated people saves 1000 uncontaminated people, well then, it's hard to argue with basic math.

I'd say the biggest risk for the general population isn't contamination from radiation, however, but from viral or bacteriological contamination. One person infected with a nasty enough virus could probably start a global pandemic. Terrorists would probably use a suicide carrier for that though. Just some guy with the sniffles traveling through a few busy airports is all it'd take.