Cancelling an order

I guess really I have 2 questions: How can the buyer cancel their order, or remove from the shopping cart? Also, if a user starts the registration process, then leaves the site and comes back later, their information is still showing.

1. How can we make it so there is a button to clear the shopping cart, or cancel order.
2. if the user leaves the site then comes back, how can we get their information removed? And, if this is a cache type issue, is there a way to clear it after a specified number of minutes.

thanks in advance,

paul