Everyone has their own share of two-faced friends and there will come a time when it will be inevitable to catch them in the act of stabbing you in the back. When you find out that someone's been backstabbing you, the knee-jerk reaction is usually that of taking revenge. This shouldn't be the case because there are many other solutions to this nasty problem.
(1) Smother them- with kindness.
It's not like you're going to turn into a doormat and let them backstab you to the point of martyrdom. It's just that sometimes turning the other cheek is still the best way to go. Don't acknowledge their backstabbing, instead treat the backstabber as if he/she is your most precious friend. Playing the guilt tactic is actually an interesting and safe solution, for if you use this card just right, in the end you'll be the "bigger man" so to speak. Take note that a hasty retaliation usually ends in dire consequences.
(2) Evaluate yourself and understand the situation.
Think about the reason why this particular person resorted to backstabbing you. Have you been too aggressive or arrogant with your colleagues lately? Is it almost logical to think that someone would take you down a notch lower in the presence of other people? Recall instances when you have stabbed other people's backs as well. Is it safe to assume that karma is working in the background and that you're being dealt with accordingly? Try to pause for a moment and contemplate on these aspects and you might just figure out a way to end the backstabbing once and for all.
(3) Interact with the people with whom the backstabber associated with.
Try and see if it is indeed the backstabbing person you must be cautious about or if it's just some kind of mob mentality (which is rather common). When the smoke clears, approach the people the backstabber talked to and start a conversation.
You know, sometimes, when there's an "idle" pause in conversations, a group of people just like to huddle up together and talk about nasty things behind a person's back. Of course, the shallowest topics may be the ones which can hurt the deepest. If you sense that it is this way, then you just have to accept the fact that you're surrounded by a legion of backstabbers.
(4) If there are no other options available, then it's time to confront them.
This has to be the most difficult solution because it's likely that your backstabbing "friend" will deny any or all of your allegations. Take a risk and tell him/her that you know all about it. It can turn really dramatic, especially if you caught the person off-guard. There might even be real tears.
To make matters simple, these solutions may end up strengthening your relationship as friends or completely shattering it, no matter how deep you perceived it to be. In the end, think of it this way: "It's better to lose the limb than let the cancer grow and spread all over."