While in the child's diary, click Food to expand.
You can see the meal times are populated with your menu.
To deselect the items, hold down ctrl or cmd and click the ones you want to remove.
Click Save.
You have now removed all options completely from that mealtime, leaving it empty to select anew.
Useful links
How do I add food to the Daily Diary?
How do I remove an item from the Food Menu?
How do I add items to the food list?
How do I create / use menu templates?