Lock billboards horizontally

[Stratium]

Something that I've been looking to do is to lock sprites to a horizontal axis. At the moment, if the camera gets above or below the sprite they'll lean to accommodate this.

This is fine for floating camera type games but doesn't look believable for some sprites, such as trees or fences, as well as first-person or virtual reality titles. Is it possible to lock individual axis, so that this can be prevented?