This will encourage them to use graphics sparingly and purposefully. Now, however, I find most of that stuff merely annoying. Graphics should enhance the use of the page. Students should be able to give three solid reasons why they chose to incorporate the graphics that they did. Next, use graphic emphasis to guide the reader through the text.
Graphics naturally draw attention to themselves.
Notice how I use this graphic to draw attention to the beginning of the page. Use graphic emphasis to guide the reader. The top of the page is where readers look first. Use the page header to capture interest and communicate purpose. And, as others have suggested, if we emphasize everything, than we really emphasize nothing.