Unfortunately there's nothing that can be done about that - it happens when someone has a huuuuuuge sig which expands the width of the page and the banner is not wide enough to compensate for the change. When someone posts in a thread and they have a large sig it stretches that page - if you look at the same thread before that person started posting, you'll notice that the page hasn't altered in width at all ^_^
If people don't want to see the stretching, they need to check out their sig and see if when they post, it widens the page. Then alter the sig size accordingly.