When you create an event or task, you have the option to turn it into a private item. This way, other users will see that you are busy on the calendar, but not be able to see the details of the item.

To make an event or task private, follow the steps below:

1) Create or edit a task or event.

2) Scroll down, and toggle "Private" to "Yes".

3) Scroll down and click "Save". 

Now, other users will simply see "Private" for the subject of the event and no description. You can safely schedule events that other users shouldn't see so that they know you are busy at a certain time.

Enjoy!