How would I prevent the player from moving/being pushed on a specific axis?

I'm working on a 2D side scrolling game, and I've run into an issue. Occasionally, when roblox physics happen strangely, the player is nudged, moving him on the Z-Axis, which, unfortunately, causes all sorts of issues with the game, as the player is unable to enter areas because of their location on the Z axis.

I'm hoping that there'd be some way to prevent a player from moving on the Z Axis entirely, and if he does manage to do that, teleport him back to a pre-defined point on the Z Axis.

If it's any help, you can see what I mean here at my place.

Thanks!