They'll keep doing it till they're kept so busy at home that they don't have time for this foreign adventurism.
By "kept so busy at home" you mean "engaging in productive trade", right?
Because it certainly wouldn't make any sense to suggest that bombing them, for example, is "keeping them busy" in any materially useful sense, since we have overwhelming empirical data that bombing and any other form of military assault has the primary result of engendering resistance.
Furthermore, "at home" is Belgium for the people involved in this action, and "at home" was France for the blasphemophobes who murdered the blasphemers of Charlie Hebdo.
You are right that this is an asymmetric war, but you don't seem aware that that requires tactics very different from bombing or other military action in many cases. Limited military assaults can serve definite purposes, as the case of ISIS shows, but the real war won't be won on the battlefield any more than the war against the Soviets was won on the battlefield.
In fact, there not being a battlefield in any conventional sense was a requirement for winning against the Soviets. Even setting aside the problem of nuclear weapons, if we had met the Soviets on the battlefield we can say with near certainty that the population would have rallied 'round the commisars, and the Soviet Empire would have never fallen.
As such, our tactical response to Islamists should be primarily--but not exclusively--non-military. It should be economic, political, satirical, even poetical: http://www.tjradcliffe.com/?p=...
It took hundreds of years for Christians to let go of blasphemophobia. It may take as long for Muslims to let go of theirs. We should be in this for the long haul, and while we should be willing to kill and die now and then, if anyone suggests those should be the primary activities involved, they are simply expressing a profound ignorance of humans, and history, and warfare (both its costs and its effectiveness, which bellicose emotionalists often get wrong.)