In the hope this helps for debugging/improving, I am sharing this here, but it might also be already known:

Take this situation:
User A shared calendar with User B

While sharing worked, thereafter:

• I could not find any option for user A to stop sharing a given calendar with user B
• logged as user A, I also could not see any trace that the given calendar had been shared and whom with.
• I could not find any option for user B to "leave" or remove the shared calendar from user A

I hope that this helps.