Once a transaction has been posted, and confirmed, it can only be completed by the members or cancelled by a moderator. To get one cancelled, you'd need to report the transaction so a moderator can look in to it. We'll make an effort to help get the transaction completed and, if we're unable to get that done, we'll cancel it.
As far as viewing cancelled transactions, that's something I can work on for the next update. Right now they're not even viewable by moderators so it'll take a bit of work to fix that. It is worth noting that cancelled transactions are completely removed after a certain amount of time has passed.