We also need this functionality and have been looking into making a module for it. What we require is the default option to read "Please select..." and for that option to prompt the user to select a proper option. I think the ideal solution would be a checkbox next to each option saying something like "default option" or "don't allow checkout". If anyone has better ideas I'd love to hear them. Also, an outline of how to do this in a module would be great before embarking on it and likely getting it wrong!