Hide details of all or some appointments and meetings
When co-workers plan a meeting and are looking at free/busy times via the Schedule Assistant, they can see the details of any appointment and meeting if they hover the mouse pointer over the item.
Is there a way to block that?
To prevent co-workers from seeing Calendar item details via Free/Busy times, you can either publish your appointment as Private or you can change the permissions on your Calendar folder.
Mark an appointment or meeting as private
If you only want to block certain individual appointments from showing their contents when someone schedules a meeting with you or opens your Calendar folder, then you can mark an appointment as private.
To do this, select the item on your Calendar or double click on the appointment or meeting to open it in its own window and choose:
- In the Tags group on the Appointment or Meeting tab, click the “Private” button with the lock icon (next to the Categorize button).
Private command on the Simplified Ribbon.
Private command on the Classic Ribbon.
On your Calendar, all items which are marked as Private will show with a lock icon in the bottom right corner; 
Change permissions on your Calendar folder
To prevent that any details of your Calendar is exposed via the Schedule Assistant, you can change the permissions on your Calendar folder.
To do this, set the default permissions for your Calendar to “Can view when I’m busy” (Exchange Online) or “Free/Busy time” (Exchange On-Premises).
Currently it is probably set to “Can view titles and locations”, or “Free/Busy time, subject, location”.
- Right click your calendar folder and choose Properties.
- Select the Permissions tab.
- In the Name list select “My Organization” or “Default”.
- Change its Permissions Level to “Can view when I’m busy” or “Free/Busy time”.
- Press OK to close the dialog.
Optionally:
Via the “Add…” button, you can add some colleagues or your manager to still allow them to see some more details. Anyone else would just see your availability times.
Exchange Online; Prevent someone from seeing your whereabouts or hints about any of your appointments by its subject or location information by setting the Permissions for “My Organization” to “Can view when I’m busy”.
In Exchange On-Premises you’ll need to set the Permission Level for “Default” to “Free/Busy time”.
