There are two ways for how to set this up, depending of your current 
iClosed setup and preferences.
Short-term solution - for specific event
This solution is good when you need to apply it to the specific day or time during a specific day. Here's what you'd need to do:
Locate event/s in your Google Calendar and click on it
Click on the "Pencil" button to edit
Depending of potential scenario:
Allowing other event to be scheduled at the time of the call scheduled in iClosed make iClosed event "Free" instead of "Busy", and hit "Save" to confirm
Blocking iClosed to schedule calls at the time of the private event
make private event "Busy" instead of "Free", and hit "Save" to confirm
Long-term solution
Utilize sub-calendars in your Google Calendar. In case you don't have sub-calendar yet, create it real quick by following next steps:
Navigate to your Google Calendar
Click in the "+" button to add a calendar under "Other Calendars"
Set the name and time zone of the sub calendar
Continue adding private events in sub-calendar
To exclude or include private events, that will be stored in your sub-calendar, to be checked against conflicts:
Navigate to
AI Scheduler → Calendar
Click on the "Edit" button in Check for conflicts section
Choose following depending of the preference:
Check the box → iClosed will not allow bookings at the times of private events or
Uncheck the box to prevent sub-calendars to be checked against Availability conflicts