In order to force delivery methods you need to change the value in the Required Delivery Method column. First log into BlocksOffice and select Ticketing, then Price Types.

Now find the Price Type you want to restrict the delivery method for and select Edit. For this example we'll be changing the Adult Single Ticket.

Enter -1 and click update to force all tickets of this type to collection at box office. 

Enter 1 to force all tickets to be delivery only. 

Leave it blank to let customers choose between collection and delivery.

Here's what an adult ticket will look like when -1 is selected -

When 1 is selected - 

And when it's left blank it should look like this -