“I think you missed the relevant part of the question which is the word “how.””
That is easy, by denying them shelter. That would then require the government to confiscate shelter and I don’t think Obama wants to cross that bridge.
I would have thought "denying them welfare benefits" would work, but I think by federal law that can't be done. Once someone sets their toe on the ground here they become a burden to the working taxpayers who foot the bill for all of these unwanted welfare leeches (not to mention the 10% - 20% actual terrorists who are being infiltrated.)