If a user is no longer an appointment participant or the wrong user is connected to a person (with secondary accounts, this sometimes happens), there is a possibility to undo the calendar connection. 

Proceed as follows: 

1. Open the page Users.
2. Press on the name of the user for whom you want to disable the calendar connection. 
3. Press the Calendar tab.
4. Press the  Disconnect Calendar button.

The calendar connection of this user has been disconnected.

These instructions are valid for the Basic version and the Plus version of the calendar integration.