My organization has recently adopted an enterprise Content Management System. For the first time, staff across 8 divisions became web authors, given responsibility for their division's web pages. Training on the software, which has a WYSIWYG interface for editing, is available and with practice, all are capable of mastering the basic tools. Some simple style decisions were made for them, however, it is extremely difficult to get these folks not to elaborate on or improvise new styles.  Examples:

    making text red or another color in the belief that color will draw readers' attention    making text bold and/or italic and/or the size of a war-is-declared headline (see 1);    using images that are too small to be effective    adding a few more images that are too small to be effective    attempting to emphasize statements using ! or !! or !!!!!    writing in a too-informal tone ("Come on in outta the rain!") [We are a research organization and museum.]    feeling compelled to ornament pages with clipart, curlicues, et al.    centering everything
There is no one person in the organization with the time or authority to act as editorial overseer. What are some techniques for ensuring that the site maintains a clean, professional appearance? 

Simon