Backstabbers need knives in order to stab you in the back..Rule #1 is do not supply them with any knives. In other words you limit conversations with them to only what is necessary and nothing more. Regarding ‘what is necessary’, you need to think carefully on how you phrase what you say so they cannot extrapolate what you said into something damaging.
Rule #2 is you need to know who else spends time talking with them and when you talk to those people you need to employ the same basic rules you now use with the backstabber. If the backstabber does not get any knives to use against you from you, they need another source for knives. A great source for those knives are other people who gossip about you. Know who they are by observing their interactions with the backstabber. You don’t know what they are saying in their conversations, whether they’re taking about your or not, but why say things to them that they may inadvertently pass on innocently to the backstabber.