Earlier quoted context omitted.
Stick them in processing centres until they can be deported. Send a clear message to anyone who might come that it won't work, you won't get in, we won't give you anything, don't risk your life or waste your money. Australia did exactly this (in the face of howling opposition) and it worked: illegal boat arrivals dropped from ~20,000 per year to almost zero. Thousands of people used to drown attempting the crossing,…
I'm genuinely wondering how harsh you'd be willing to be to get what you want. What would you do if an individual can't be deported because no country will accept them? Or if their country of origin is likely to kill or torture them? Or if no commercial carrier is willing to risk operating to that country? Would you be willing to deport unaccompanied children with no guarantee that they'd be cared for?
There is a village A dragon comes to the village every year. In exchange for 2% of the children, it spares the rest and promises its “magical” protection from unseen enemies. This arrangement has lasted 2,000 years. Most villagers worship it, even though the custom has left their village far worse off than others in the land.
Some villagers move away. Not all of them are dragon-worshippers, but some are and they still try to summon the dragon.
Now the dragon free villagers face a choice:
Keep them out. But that means some innocent children among them will die.
Let them in. Risk the cult spreading again inside the walls and possibly bringing the dragon back.
Go kill the dragon themselves. Accept substantial casualties including innocent dragon worshippers and some of their own people.
Killing the dragon would mean temporarily brutal treatment of the worshippers and the destruction of their culture, but it would spare future generations from an unbounded amount of suffering.