Prevent? HA. Laughable. United gave SWA 19 city pairs out of EWR with the merger cause they bitched so much to the DOT. And a clue for you here, just because you have an airline, it doesn't mean you get to fly where you want, when you want, even at others expense no matter how much you jump up and down and cry.
United "gave" SWA nothing. They sold us some EWR slots of their own volition--part of changing their business, I suppose--and in addition were forced to divest other slots as a condition of their merger with Continental (being a slot controlled airport, and all). The government then auctioned off the slots, and SWA bid the most. Sounds like normal business operations to me.
As far as "jumping un and down and crying," you need to look in a mirror. (or just reread the earlier HOU discussion regarding Southwest working its own business model at HOU, and Unical whining and crying and trying to interfere, so as to prevent actual competition.)