Block availability when busy

Busy times in your Google Calendar will block availability in your website.
Go in Settings > Google Calendar.
If not already selected, select the linked Google Calendar - this is the calendar from which the busy times are read.
Activate the Block availability when busy switch, so any event in your Google Calendar that is marked as BUSY will block availability of your services.
This allows great flexibility, as you can still have some events in Google Calendar that you may choose not to block availability of your services, just by marking them as FREE.
Where to set a Google Calendar event as busy/free? Just edit the event and choose its busy/free property.