If you use an existing calendar, you can set up Breely to block off times from that other calendar. Google, Office 365, and iCloud can all sync to Breely under Features & Integrations > Calendar Syncing.
You can also block off time directly in Breely. This can be done by:
Clicking and dragging on the calendar.
Choosing the "Block Time" button under the mini calendar on the right of the Calendar page.