We have a large, 4,000 seat venue who only uses seating charts. They block off seats which they will distribute to customers later. These are reserved because they cannot be bought online by customers, and they are not selectable, so no one will take them.
How we achieve this, is by creating a new "special" ticket type. We called theirs "Not Available Online" and we set the number available to unlimited. We also set the available dates for purchase, to some date far back in the past. This will make the ticket type never be available.
Then, in the seating chart, we simply apply this ticket type to any seats which we want to reserve and hold off, so no one purchases it.