It is the duty of all those who wish to see international order maintained to do everything in their power to disrupt the flow of such people.
All conflicts are ideological and this conflict is no different. We require political and religious leaders in the region to be much more vocal about the fact that this has nothing to do with Islam, that it is a cruel, barbaric, mediaeval and misogynistic creed, and that it is not religion but a political perversion. We also need to make those messages clear to those young, impressionable individuals in Britain who may be considering becoming involved in such an enterprise. Those who are already there need to understand that they are not welcome back in this country and that the full force of the law will be applied should they come back. They cannot take a jihad gap year and come back to the UK with impunity.
The question of oil has been mentioned but, through the international financial system, we also need to stop financial flows to ISIS. It is very well funded and we must stop groups in the region playing a double-game, publicly decrying ISIS but providing it with the funding it requires.