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.