1. Make the split uneven. For example for every dollar advertiser pays, only 30 cents goes to the page owner who received the "Likes"
2. Lobby to make these kinds of activity illegal. They are after all spamming activities
3. These kinds of fake account farms will usually have very few connections to outside world. That is, accounts that do not obey the "Six Degrees of seperation" should be flagged for human review.
4. Once reviewed enlist law enforcement help to track down the owners of the all the accounts in these farms.
Painstaking work, but I guess the point is that advantage can lie with Facebook as opposed to the spammer in this game. I am not sure about the ethical implications though.
Maybe this is why Google wants to succeed badly in the social space : extra spam fighting abilities.
I couldn't find any resources on how Facebook handles this.
This definitely takes non-trivial effort, but it's also definitely doable if you have a serious interest in perpetrating click fraud. While Facebook hypothetically could fight this form of click fraud, it'd likely be difficult and the method would be fragile/easily circumvented. Like most things, PPC advertising can only function in a world primarily populated with positive actors.