You can not delete a show from the history of ondemand. When you are viewing on demand, the show has a lease time for it to be accessed - usually 24-48 hours or so. Once the time expires it will no longer show as in progress. The only thing you can do is wait out the time.
I have over ten years experience in the cable and internet business and and highly experienced in home wiring and issues associated with that as well as computer issues and networking. Also very familiar with the cable plant and issues associated with that. I have over 6 years experience as a network maintenence technician. My study in College was computer science and very familiar with windows up to Windows 7
You need to go into the list option. and using the left and right arrows look for the record log, should be pretty straight forward. the shows that are already recorded will appear in your list. You can just select the show and should have an option to delete. Also using the left and right arrows you should see something with all your recording shows by date or time you can cancel them their as well but they will not record if you do this