They can, but they lack the will. It's more an issue of Western countries being unwilling to stop them for ethical reasons than being unable.
I mean, we could go in and kill every person living in these countries indiscriminately with bombs if we were determined, and that would kill ISIS. But that would be ethically and morally unacceptable to most people. And they've nestled themselves amongst non-combatants so completely and thoroughly in a spread out fashion that the only way to be sure of getting them all is to destroy an entire population of people. So it's possible but unrealistic.
Another possibility would be committing ground troops, but that's also unrealistic unless there are several more 9/11 style terrorist attacks that kill thousands at once. Democratic countries are very reluctant to commit ground troops.
It would probably be more desirable to simply contain ISIS and keep their operatives out of Western countries.