Some advocate diplomacy (talking), but that only works with rational leaders of an established nation.
When you have a rogue nation or a group of people who are not fighting for a recognised country (like IS, Al Qaeda, separatist groups fighting to overthrow a government, etc), there is no rationality, compromise or element of trust.
IS/Daish has the primary goal of eradicating all non-believers in the entire world, either by converting them to Islam or killing anyone who won't convert, because that's their misguided interpretation of Islam. (800 years ago some Christian leaders had the same misguided interpretation of their own religion, believing it was their duty to wipe out the inhabitants of the middle east to create their own holy land, and hold inquisitions to make sure the people were dedicated to the faith... but that was a different time, called the Dark Ages for a reason, and the mainstream Christian religions are not like that anymore - they now focus more on preaching love and forgiveness over fear and punishment)

Others advocate staying out of the conflict because it is not in our part of the world, but as we've seen already, people from all over the world are being indoctrinated to either fly there to fight or carry out acts of aggression in their home country. And it will get a lot worse if IS are left to establish a safehaven area or are given their own lands (country) to act as their base of operations to carry out their primary objective - to eradicate all non-believers.

War is never a good thing, but anti-war advocates and protest crowds need to differentiate between an opponent that can be reasoned with to prevent conflict (like the cold war preventing a nuclear war), from an opponent who is willing to die before they compromise, surrender or cease hostilities (like these state-less groups).