Press the YouView / Menu button on your remote, select MyTV and then Scheduled.
Use the down arrow on the remote to Scroll down, select the programme, press Delete on the remote. The deletion doesn't give (or require) confirmation. The programme thumbnail will be removed from Scheduled recordings.
Or via the Guide, find the programme in the future part of the Guide (it will have a red circle with white R, a Record icon, just like the Record button) and press the Record button on the remote. There will be a confirmation, top of screen, that the programme will no longer be recorded and the Record icon will disappear.
Note that the complete series is cancelled as it's not possible to cancel just one programme in a series.