Bleeding them through continuous bombings and “droning” their leaders, while at the same time walling them in until the only people they can kill are their own, is probably the best solution.
It is quite a long article, but if you are really interested in understanding the mythology and psychology that drives the Caliphate, it is definitely worth the read.
Excerpt from The Atlantic “What Isis Really Wants”:
“For certain true believers—the kind who long for epic good-versus-evil battles—visions of apocalyptic bloodbaths fulfill a deep psychological need. Of the Islamic State supporters I met, Musa Cerantonio, the Australian, expressed the deepest interest in the apocalypse and how the remaining days of the Islamic State—and the world—might look. Parts of that prediction are original to him, and do not yet have the status of doctrine. But other parts are based on mainstream Sunni sources and appear all over the Islamic State’s propaganda. These include the belief that there will be only 12 legitimate caliphs, and Baghdadi is the eighth; that the armies of Rome will mass to meet the armies of Islam in northern Syria; and that Islam’s final showdown with an anti-Messiah will occur in Jerusalem after a period of renewed Islamic conquest. …
Given everything we know about the Islamic State, continuing to slowly bleed it, through air strikes and proxy warfare, appears the best of bad military options. Neither the Kurds nor the Shia will ever subdue and control the whole Sunni heartland of Syria and Iraq—they are hated there, and have no appetite for such an adventure anyway. But they can keep the Islamic State from fulfilling its duty to expand. And with every month that it fails to expand, it resembles less the conquering state of the Prophet Muhammad than yet another Middle Eastern government failing to bring prosperity to its people.”