How to motivate speakers to give less specialized talks at a "colloquium?"

Our department is beginning to have a serious attendance problem at our colloquium. The appropriate level for our colloquium is that any person with an undergraduate degree in a quantitative/theoretical field should be able to follow the talk as long as they are paying close attention. In a letter to our speakers we say "Your talk should be entirely self contained. You can assume the audience has a basic understanding of undergraduate real analysis and linear algebra but do not assume any prior knowledge in physics, biology or more specialized mathematics. Of course feel free to talk about such topics, just be sure to explain all basic concepts needed to follow the rest of the presentation as soon as they come up." Despite this warning, speakers go on, as usual, giving seminar level talks. Graduate students and advanced undergraduates have basically stopped attending, which looks bad on our department, and lowers student morale.

Besides adding the request in our email, that the talk not be too technical, is there anyway to prevent speakers from giving talks that are too specialized or advanced?