For any day where your resource's working hours would normally set them as available but you don't want them to be available, you can add a day off in the resource working hours:
- Hover over your business and select resources
- Click edit working hours next to the appropriate resource
- Go to the bottom left of the working hours page to find days off
- Select the date on which the resource should not be bookable
- Click add day off
- Repeat for multiple days off.
Alternatively, you can add blocked time in the calendar - find a guide here.