Although direct and firm intervention is always required when dealing with an ex’s attempting to alienate you from your child, always keep this in the back of your head: the concept of the Brainwashing Boomerang.
Brainwashing Boomerang (n) – when a parent’s attempt to alienate their ex from their child or children backfires, and the intended effect is the opposite: The child or children resent the parent who attempted the brainwashing.
You can’t sit back and wait for this effect to take place down the road. It is something that often happens, however, especially in cases where the children have relatively strong and independent minds (ie, less susceptible to mind shaping). This blog doesn’t recommend this passive approach, however, as damage is still done to the child’s psyche.
And even this boomerang effect isn’t desirable, as it simply switches hostility between the two parents… in other words, hostility which shouldn’t be there at all still exists.