Can we block certain time slots and/or choose which time slots are available? Not just on a weekly basis, but for each individual day?
Yes, you'll be able to block out individual time slots and individual days as well.
1. You can navigate to Inbound > Call Reps > Working Hours
2. Click the edit button to set availability 
3. Once you click, pop-up working hours will appear
4. For instance, you want to block Wednesday and Friday, set up the setting as follows. Click Add New to set your specific availability
5. Next, determine how many days it will be available under Schedule Days. It will help you to create recurring availability on your booking page. Go to Call Setting > Schedules
6. Once you're done, click the update button
The booking page will look like this
PS. You can block certain dates as well through google calendar if you integrate the widget with your own Gmail account. Novocall will automatically check and sync to your callback widget.