Bounding width when we give users' TinyMCE or FCKEditor?

I finally got TinyMCE to work beautifully and now am trying to
understand how I will bound my users' from not pushing the HTML beyond
the borders that I grant them.

For example, if I type;

9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999

It goes off the edge of the <div> space I have defined for this
element.
Is there a way to still grant them the HTML they produced as a result
of using this editor and keep them from going off the edge?

Thanks,
David

What I have is something like this in my css:

.post-message {
  word-break: break-all;
}

That does it for me...