I don't know if this will help with an older child, but when my 3-year-old DD stops following the rules, I stop playing. I tell her the absolute truth: that I do not like playing with someone who does not follow the rules.
Agree with the pp. Also, Im sure you've done this but emphasize to her that if she cheated to win then she didn't win at all. So if she gets excited about a win that you know she cheated on, deflate her sails a bit by refusing to acknowledge any "win" and point out why she actually did indeed lose.