In quite a few discussions on the topic of the stopping 'boat people', my usual go-to answer is snipers.
I very rarely get asked again, which is a shame as my alternate choice is Minefields
Of course this is just me being the fearless and intrepid dick that I can be, because if I say "you'll never stop them" it just provokes further pointless discussion.
The monster is out of the box and growing, and you won't get it back in unless you slice and dice it.