I don't remember on which radio station I heard it, but there was a former Green Beret who said what you said about ISIS...that it was more an ideology than a state. His thoughts were to allow troops to engage with them on the battlefield and give them a good ol' fashioned butt kicking, then only stick around long enough to curb stomp them a bit more, just for the sake of humiliation. Then leave. No nation building, no financial aid to rebuild; give them nothing. If they start trouble, come in and humiliate them again. Repeat as necessary.
He pointed out that it won't kill the ideology, but what it would do is cause fewer and fewer to join the ISIS die hards each time there is a humiliating and crushing defeat on the battlefield. His belief was that eventually - and it wouldn't take too many campaigns really - the rest of the area will get tired of the ISIS types and the problem will take care of itself.
I'm not sure how effective that would be, but it would be something that would expose ISIS as the cowards that they really are, as they would have to fight or shut knock off that crap. This Green Beret said that ISIS is a bully and should be treated as such.