Assign a security detail to the stadiums with Stingers
Have them in radio contact with ATC to give as much warning as possible. Stadiums are already restricted airspace during an event, so let all pilots know that any plane that flies within 3 miles of the stadium will be fired upon without warning. Private planes are very easy to steal. Their door locks are a joke and they're easy to hotwire. An explosives laden light plane could kill hundreds, perhaps thousands of people. This is no time to play around, and I write this as a private pilot who has considered this possibility ever since Sept 11th.