Sure, for that we will need an other function that will be called when you click the checkbox with cheese. It will also tell the function weather or not it's checked (so it will disable them when you click to check and re-enable them when you click to uncheck it) :
<input onclick="cheeseonly(this.checked);" type="checkbox" name = "size" id="cheese" value = "cheese"> Cheese Only <b />
Sorry I didn't see the edit you made in time, check if the way I posted here works as well enough, if you insist to have the alert it's easy to add but you should know alerts are best to be avoided since they usually cause discomfort to your visitors.
Yeah the above code works, but could you show me how to make it so the alert pops up ONLY if there are other checkboxes checked and it also makes them unchecked? I know it is annoying but I wanted to see what it looks like first.