Depends on the case.
If they are really sorry and have seen the error of their ways, give them a lot of taxpayer money....
....to tell others about what a bad idea going there really is.
I still don't know that many people who are stupid on purpose / by choice. It's probably better when a returnee tells them not to go than to send a boring government employee in a beige suit with colourful leaflets.
Of course if they did a lot of horrible things back there and/or aren't even sorry and come back for other reasons, jail them for treason or so.
And since I'm not sure where else to post this and it's somewhat related, here is an interesting interview with someone who went there and still is there. The journalist made a deal with the IS over the internet and was allowed to visit the state relatively safely. Still took a lot of balls because he couldn't be 100% sure they'd stick to the deal. It's German with English subtitles.
https://www.facebook.com/video.php?v=10152723644955838
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