The very best response imo - not that this will ever happen, France just bombed Raqqa - is to do absolutely nothing. To deny them a platform and to very quietly concentrate on the ringleaders and the process of radicalization. This will take a lot more guts than what we collectively have because it would certainly lead to more attacks as the counterparty gets more and more desperate why their ploy isn't working but the Achilles heel of ISIS as far as I can see is their ability to recruit. If they are judged to be unable to move the needle in any meaningful way (either negative or positive) then they will lose. On another note, kick out each and every Imam that preaches hate to their flock rather than compassion and balance. Those guys are definitely part of the problem and they should not be aided and abetted because they are effectively part of the recruitment arm of ISIS. On a more positive note, this seems to be changing and these characters seem to be much less welcome than they were in the past. Yet another note: joining ISIS by westerners should be a one-way ticket, once radicalized and gone to fight there there is no way back. This would require quite a bit of legal hassle.