Double nationalities is relatively easy - they should loose their (usually) Western one when they decide to prance off to the 1500's.
Those without - a lot harder. Part of me thinks that the point of the MI6 etc is to collate evidence and ensure that they
don't come back. Just quietly killed in battle rather than monitor them across the whole of Europe to then have to spend money on due process. We're not talking about collaborators in an occupied country where people play lip service for safety, they've spent a lot of effort to get to Paradise and I say we help 'em.
Otherwise, there's nothing we can really do. There will never be the evidence required to try them (as has been pointed out, circumstantial isn't hard evidence) and so we can flap about worrying about the few that do decide to become sleepers and spend loads of money integrating the others.
All we ask in the West is that you join in with the culture we have and most places in Europe have a very broad definition of what this is and practically all cultures and religions manage to do so - bar a very small minority of Muslims. I don't say what we do is right / the best but it is what we do and is our norms. Rather than continue to spend resources on those who clearly don't want to be, get rid of this few and spend the time integrating everyone else.

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