I don't know who it is exactly that you are disagreeing with, but it isn't Dr. Sowell. Sowell NEVER made the proposal you are claiming he did.
Quite to the contrary, Sowell made the explicit point that it would NOT be neccessary to round up and jail them all.
Instead, he rightly noted that if we started actually treating these people as criminals, subject to arrest and trial, many would repatriate themselves forthwith. Take away the incentive, put in place some serious disincentives (like enforcing existing law), and the busses south will be filled to the brim with folks looking for a new life in Mexico.
Actually, I think Dr. Sowell stated that officers of the law were told not to report illegal aliens.