Get rid of Ghost-Notebooks

Hello! I do not know if I am in the right place and I apologize directly for my ignorance!
We use Office 365 at school. When I create a new group in Teams, use the notebook there and then delete the group later, it can still be found in the list of notebooks - but it can no longer be opened. The list is therefore totally confusing. Hiding then does not help. How do I get rid of these ghosts?
The admin doesn't know either...
Thanks!