On an nvidia card with Detonator drivers, it's under "scaling" or "monitor scaling" -- basically you want to set it to NO scaling
Then you find the right resolution to play AH under, so that it takes up most of the vertical space.
For example: Say your screen is 1600x1024, the next best 3:4 ratio is 1280x1024. Say your screen is 1280x800, the next best would probably be 1024x768 (leaving a little black space above and below as well, but not as much as on the sides).
It will center the pixels on the monitor, whatever resolution you choose. I've done this with older games that run in lower resolutions, out of curiousity.