Lets face it, once they are across they won't get sent back so we may as well cut out the middle man and ship them in ourselves. At least that way we are in control.
The only way to stop it is to stop it being so attractive in the first place. If, in effect, they are given free food, clothing and housing once they are here, it is not a great deterrent.
Perhaps the large sums of money that government spend on them should be spent on making their original places more enticing, that way they wouldn't need to leave.
Of course we all know that if that were the case, most of the money would be syphoned off anyway and things would be no better...