Piece of cake: just move the fence a couple miles north to isolate those who oppose it's construction, making sure they're left on the "Mexico" side.

I don't see a border fence as being any different than any other construction project, be it a freeway, railroad tracks, or just a new sidewalk - if the government needs the right of way and/or the land, they'll either 1) pay you for the land and build the stuff, or 2) bury you in paperwork and legal fees until you cave.