I setup a sharing policy to allow my users access to share their calendar using their OWA account. They have sent this calendar to a few clients. When setting it up they didn't have any granular controls for how long to share the calendar, only the option to choose the level of details they want to share.  The share worked correctly, however, they want to cancel the shared calendar and do not know how.

How can I proceed to have this rectified?

Go to OWA -> Calendar -> right click on your Calendar -> Permissions. You should have a section for internally shared and externally shared.

Hi,

As what Vasil L. Michev says, we can remove the calendar permission for the external organization users in Outlook 2013 or OWA.

If you are using sharing policy for calendar sharing with external organization, please disable or remove the sharing policy in Exchange server:

Set-SharingPolicy -Identity "Contoso" -Enabled $False

For more information about Modify, disable, or remove a sharing policy, please refer to:

https://technet.microsoft.com/en-us/library/jj657460(v=exchg.150).aspx

Regards,