If you want to remove all items from the users carts, you will most likely have to write a custom function that loops through all of the users/customers and cleans the cart out. What is the reason behind you wanting to do this?
Just as a reference. So first you would need to get the users/customers cart in the loop, and then call that method on the shoppingcartinfo obj that you get from the ShoppingCartInfoProvider.GetShoppingCartInfo(UserID,SiteName) method, and then call CleanUpShoppingCart() which deletes the cart from the database and session. Just beware that this can't be reversed without a database backup, so I would make sure that this absolutely has to be done. Maybe explaining why this has to be done can help us determine if that is the case.