As far as I have found there are only 2 ways, both not practical:
- Wait till your sandbox lifespan is over. A new one will then be created automatically on most pages of where the sandbox is used.
- Get into another learning module which uses a sandbox and it might see it as a different sandbox, therefor cleanup the old one.
I can only sincerely ask Microsoft to improve this sandbox implementation, since with subscription disappearing at random moments in my lifespan this is not a pleasant way to learn.