You need to find out what is the exact aspect ratio that the codec uses. After you do that you then need to fit the window in which the video is reproduced to the same aspect ration that the codec needs.

So, for example, if the codec uses a fixed aspect ratio of 4:3 then you need to set the dimensions of the window in which the video will be reproduced to an exact proportional aspect ratio, like 400x300 pixels, and this will solve your problem.