A better alternative to deleting all default items from a category is to just hide that category and create a new one. To hide a category, go to the category view as shown below and tap on the menu button and select "Edit Categories". Now tap on the red minus button on the left which will bring up a Hide button to the right. Tap on "Hide" to hide that category. Note that hiding a category will only affect your current shopping list's catalog; so for example you could hide category "Vegetables" from shopping list "Sports Authority" and that category will only be hidden for Sports Authority shopping list but will not affect other shopping lists where Vegetables category makes sense like for "Costco" or "Shoprite".
You can now add a new category and start adding your own custom items to it.