Billboard GUIs lock the camera - How do I fix this?

Hello. Recently, myself and other people who play a game I develop for realized that when the mouse is hovering over a billboard GUI, all camera control is lost.

What do I mean by that? Well, you lose the ability to move the camera with the mouse, along with the ability to zoom out/in with the I/O keys. The only way to escape is to open the [esc] menu, or to move your characters so that your cursor is no longer over the billboard GUI button. In 1st person you can’t move your view, and in 3rd person it freezes your cursor.

How do I fix this? Why does this happen?

Also, bonus: I can only zoom out using O when I am not already zoomed in all the way. In first person, the O key does not do anything.