While I'm inclined to agree with them about that, I'd say the very offer proves they can not be trusted, so realistically my suggestion, which would work, would be to literally level every building in Gaza and every person, one by one, until Shalit is found. While many in the world would complain about this, the words of the wrong are simply not worth worrying about. No one in Gaza would be left to complain, if Shalit is not found by the end.
Australia's most dangerous Islamic State operative killed in U.S. strike (AP)
from the article: Palestinian prisoners' families to urge Hamas: Don't surrender to Israel