wp-ecommerce clear cart

When speaking about limited quantity…I have noticed that when a user adds something to their cart but then does not proceed to checkout and leaves the page,then when next user trys to buy that same item, they cant. It says out of stock. Technically it is becasue the last user put it in thier cart.

So my question is, how do i “clear all carts”, so that next user can make the purchase???