Go to "Registration" from the blue navigation bar. In the Registration window select "Pending" to view your registrant who opted to pay via an offline method. Select the registration you would like to cancel by clicking the registrants name.
Once you're looking at the Registration Detail, click the gear button on the far right and select "Cancel". You will be asked to confirm that you want to cancel the registration.
NOTE: Message Blocks limits the number of offline payments that can be cancelled per event. If you need to cancel a large number of registrations, please email email@example.com