Using SBS2008 and Outlook 2010, 07, and 03. Users who have Outlook 2010 are not able to delete calendar items off of shared calendars. I have verified permissions are correct - the same user can login to a PC running Outlook 2007 or earlier and delete the calendar item. Error message displayed is: "Could not complete the deletion. The items may have been already moved or deleted."
Unfortunately, not the solution. This is universally impacting all users on the network using Outlook 2010. But only Outlook 2010. The same user can login to a PC running Outlook 2007 and delete shared calendar items without issue.
See this article, which, though it does not specifically reference Outlook 2010, does include a possible resolution of this issue:
Go to HKCU | Software | Microsoft | Office | 14| Outlook | options | General
You will see a value named DelegateWastebasketStyle with a value of 4. Change it to 8 (this will tell Outlook to discard these items in your own trash account instead of the user who is sharing their calendar), and it may resolve your problem (as it did mine).
You will have to restart Outlook to have the registry setting take effect.