Open your event with reserved seating.

Click 'Seating chart' in the left-hand menu.

If you're using recurring events, choose the day or time you want to hold seats.

You'll see your seating chart. Select the seats to put them on hold.

Choose 'Apply changes'.

Now, your seats are on hold, and off sale.

You can apply up to 10 changes per day. You can't increase the number of changes you can issue per day.