How to constrain a Sphere CollisionShape to 2 dimensions?

[erib3]

If I apply external force on 2 axis, after running the simulation for a while I get movement on a 3rd axis. I am not sure where this energy is coming from.

I have looked into constraining the object to 2 dimensions but it's not clear how to do this in pybullet. What is the simplest way to express that an object can only move on the X and Y axis?