To block out a section of time on an appointment calendar, first check that you're viewing the correct calendar.
If its the venue you need to block out, select the venue.
If its a specific staff members calendar choose staff > then choose their name.
Then click any open space > use the time blocking toggle at the top right of the pop up > choose whether to make this a recurring block > add the time and the optional description > Save.
Your time block will look like this.