Why isn’t overstaying a visa an automatic 10 years in prison?
Freeper Oldeconomybuyer's suggestion is much better.
Why bother with retribution while having to feed, clothe, house, and provide medical care for them? Put them on the slow boat back to wherever they came from after fining the bejezus out of them and be done with it. The feds need to prove they can handle these open and shut cases while devising an approach that quickly, humanely, and legally cleans out the barrios of the illegal entrants.
Don't worry with the physical wall as much as the wall of legal challenges for employers that enable this crowd to stay here. Self-deportation is the first step to thin the herd while saving resources for the tougher fight.