How do I block seats so they can not be purchased by the public?

Blocking seats can easily be done by logging into your account, and from the Box Office page select the seats from a reserved seating chart by clicking on the performance date.  You will select individual seats from your venue and hit CHECKOUT. 

On the next screen under Payment Method, choose Reserved, and hit CUSTOMER INFORMATION.  You can then enter the information for the reservation (first and last name will be mandatory; address, email, phone, etc., are optional).