Ok, the user has a 2.3mh rig on the site for rent (offline) so I doubt they are going to complete the rent, probably just wanted to test the system and mine was the test subject.
Maybe you could change the 1hr period to 30mins or even less. I don't think many people who are genuinely going to rent will wait 45mins before paying, and even if they did they could always pick the next server. 1hr weights it towards the renter...
Or alternatively give the leaser a cancel button to cancel a rent as long as payment hasn't been made...