What to do with the returnees?

[Fragony]

All western citizens who wish to remain western citizens should not travel to the Islamic State. Anyone who is found there can be assumed to either a sympathiser of said state, in which case they can give up their western citizenship and become a citizen of the IS. Or they are travelling there on their own accord. In which case, hand them over to the IS and let them do with them as they will. If they want to get along, let them become citizens of that state, while if they don't want to get along, wash our hands of them. The only exceptions are anyone explicitly and formally authorised by a western government, signed off by the head of the government so there can be no argument.

It isn't all that easy in some situations, the IS is not a recognised state so there are some complications.

Different question on this subject, what to do with refugees from IS territory. Who says that who is declared dead is actually dead.