So, lets see. The answer so far is to put armed people on every roof in every city.
Cool...
Jeff is right. The real answers to this conundrum lie with the Muslim community. They should form a task force and route these bastards out.
One of their (ISIS/Al Qaeda) 'War Aims' will be to engender and promote civil war and we must be extremely careful not to allow that to happen because it would be a war that would signal the end of our democracies and personal freedoms, with only a doubtful eventual outcome over many years.
You've seen what can be achieved in our free democratic societies with a few determined men, in some cases resulting in the paralysis of a whole city and economic structure. It will be very difficult to combat those tactics using conventional means.
Joined up thinking required here, principally from the Islamic communities, after all, many millions of them enjoy the relative peace and harmony of Western civilisation. I'm quite sure they would not want to exchange their suburban ideal for downtown Aleppo.
But hey! We could just give everybody a gun!