Camera Controls - Lock axis to prevent rolling?

Using the camera controls here:

I've been struggling with figuring out a way to lock the horizontal axis with camera controls such that the user doesn't roll when looking left and right, and wondering if anyone has figured out a way to overcome this?

I want to start the user directly on the ground level, so I don't have to worry about having the horizontal axis locked as they zoom in... I have that setup, but currently when I try to use the mouse to look 180 degrees you start to roll about 90 degrees and gets worse as you continue to rotate.

Anyone have some ideas?