Disallow specific combination of checkbox selections in term reference field? I have a term reference field in the format of checkboxes. However, I'd like to disallow certain combinations of checkboxes. The options are:

[]None
[]Food
[]Drinks


The "None" option is not the default none, but a taxonomy term that is actually stored in the database. I'm wondering how to allow two selections, but to disallow the selection of both "Food" and "None", or "Drinks" and "None", since in practice that really doesn't make any sense. Any suggestions?
Take a look at the Field Validation module, it attaches a UI where you can set custom validation rules for each field on a content type. 

I am not sure whether any of the inbuilt options will work for what you want, but you can select the PHP code option and write the rules you require. There is a short reference for the PHP validator here: http://drupal.org/node/1537028