Constrain aspect ratio does exactly that. However, you should not adjust the actual rendering resolution when using it.
So if you for example want a square aspect ratio on a HD monitor you should leave your resolution at 1920x1080 and the engine will add black bars on the sides to correctly letterbox it. If you instead set your resolution to 1080x1080 as well the engine will think that no letterboxing is needed and simply output a 1080x1080 image. How it is then displayed depends on your screen and graphics card settings, which tend to default to stretching the image.