Here's an old thread on the subject:
http://ranchers.net/forum/viewtopic.php?t=27154
You'll never keep up with them if you try to hunt them. Do like efb says and trap, trap, trap. Get your neighbors to trap - this needs to be a group effort. Most people don't realize the seriousness of the situation until it's too late. In fact, if you've already seen one, it's probably already be too late.
Trap, trap, trap and never let a live one get away. Kill every feral hog you get in the trap. Feed out the young ones if you want to, but NEVER let one live after he's been in a trap or nobody will ever be able to catch him again.
The only good hog is a dead hog. Kill every hog you can and if you hear about people turning them back out to hunt or selling them to people to hunt, go ahead and kill them, too.