arrest them, put them in camps and rent them out to US farmers that need/want cheap labor.
you see it is a win win, the aliens get to stay in the USA and work , which is what they want. the farmers get cheap labor, which is want they want.
now you say "how will you arrest 30 million?", you don't have to arrest all 30 million, you arrest a few and the rest will get the hint and run home.