If you want to stop online bookings but still allow internal bookings there are two ways:

1) Create a Periodic Close out

Follow this guide on Periodic Close Outs (Recurring) 

2) Exception Date

In Settings > Restaurant Details > Other Details > Booking and you can add an exception date:

Add in your dates

Click Create Exception

Remember to save at the bottom of the page!

You can also remove them by pressing the x next to the date.