...or (but you may believe this "disfigures the page design"):

You can identify elements of excessive length (for whatever value of 'excessive' works for your ap) using length ([perldocdoc://length]), and then handle such cases with CSS2's "overflow" (CSS3 also offers "overflow-x).

For CSS2, see http://www.w3schools.com/Css/pr_pos_overflow.asp which offers an example similar to this:

div
{
width: 240px;
max-width: 240px;  // Belt and suspenders. Rarely needed
height: 200px;
overflow: scroll;
}
[download]

Div's with this styling will add vertical and horizontal scrollbars, perhaps calling visitors' attention to the run-on lines more effectively than the horizontal scrollbar at the base of the browser window.

As a very rough rule of thumb, for ordinary fonts, your 40 chars spec translates to a width of about 240px (but you may need to reduce that, depending on your design, to allow for the vertical scrollbar width.