Using "Horizontal Span" makes the two monitors behave as if they're one screen, which is why windows maximise across both and center themselves over the split. But maybe you're talking about some sort of options I've missed? Or maybe we're using different versions of the drivers...?
You're sure you're not thinking of the "Dual View" option? If so, yes it will stop those things from happening, but it'll also stop you from being able to spread a 3D app across both monitors.
You're sure you're not thinking of the "Dual View" option? If so, yes it will stop those things from happening, but it'll also stop you from being able to spread a 3D app across both monitors.