This is a shot in the dark because I can’t see the site… Check the way your theme uses css to align your sidebar and see if the width that is being set to the div by the user is causing the post container to become too wide, pushing the sidebar down. You could use some css to control elements within the posts container such as:
I would try to find the root in your theme’s css or template design. Maybe you are missing a closing .
Well, hopefully this sparks something that helps you resolve this. If you can post a link we’ll be able to figure it out for you faster.
*EDIT 4-23
I would say the protection for something like this is not a feature of WordPress but dependent on the theme in use following template design guidelines. ANyone skilled in inline css and html can break any template using the post or page editor. There is such a variation of templates out there that are not designed by reputable WordPress theme designers that it is hard to say what the root issue is in your instance but I bet it lies in the theme’s design combined with the inline html placed in the editor. I would wager that a more well designed template would be built in a way that makes this harder to occur. Using standard cross browser and device compatibility rules will go a long way in preventing this kind of thing. Are you using a particular theme that I can download and test against my hypothesis?
*Also try adding this to CSS and it may resolve your issue:
iframe {
max-width: 100%;
}