To unassign a resource you will need to be in the Reports section of your chosen product (Reading Planet, Cracking Comprehension, or Achieve).
You can filter your reports to a specific resource or pupil group:
Once in your chosen products Report, you will be able to 'Hide graph and trouble spots', to expand the list of pupils which appear on the left hand side.
To open a pupil report please click on their name, in this pop up will be able to see their Achievements and any remaining books/quizzes to take, click on 'My Books/My Missions' and here you will see the option to 'Remove':
Please note: you can only remove resources individually from a pupils account, you cannot currently bulk remove