If there's a table available on the day's schedule that you may need to block out for special purposes, you can block it out without having to create a new sheet for the day(s). Just find the "Create Block" button on the Floor, Book, or Grid tabs in the sidebar, then enter the start and end times, date, shift, room, and table in the fields that pop up.
You don't need to create a block if your purpose is to hold the table for a special reservation – for that you should just create the reservation and add any reservation notes or custom reservation tags as needed.