Obviously to preserve the correct aspect so that things are not stretched sideways to fill the redundant area on the two sides - thus making people appear fatter, you will have to crop the height of the video.
Whilst you could simply crop the entire video from start to end this would result in tops of heads being cut off in places or important items from the bottom being cut off at other locations.
You have to employ one or more different techniques that will vary with the actual content.
You can do one or more of the following.
a. project the video onto a false background like I did when I converted my collection of Super 8 cine films to DVD.
Converting Super 8 Films
b. split the video up into several smaller clips and apply the cropping filter plus the use of key frames to "follow the action" thus keeping the important part of the video on screen whilst discarding the area(s) that are less important.
c. Use the Distort option to resize the video.
d. Use split screen effects