I suggest you contact
ceo.office@virginmedia.co.uk. They can investigate what is happening and, as jb66 says, block On Demand.
You can also change some settings on the Virgin box yourself. Go into the Parental Controls and/or Purchase Controls menus (the menus available will depend on the box type), and set everything to the maximum. Then change the PIN to a random value instead of the easily-guessed default of 1234.