Deselecting a drop down box

Hi

In my business i create and edit diets for people.

I have a form that creates a diet via drop downs linked to foods which is then submitted to a database and a email sent to clients of that diet, This all works fine.

The issue is i review these clients weekly and amend their diets on how they look. i open their diet and i sometimes want to remove items i put in. example: I create a diet for you and tell you to eat rice then next week i want to remove it. how do i set that drop down back to being blank? and re submit the diet to dataset.

thanks

Fraser