Performances can be taken off-sale:

a) on the web so they are only accessible via Box Office 

b) on both Box Office and on the Web so they are inactive and not on sale anymore. If there are existing sales that need to be shifted on the performance you want to make inactive, make the performance box office only at first so you can still see them and see here for more info on moving bookings and transfers:

How to transfer booked seats to a performance or session of another production or event

Simple Transfer With No Booking Conflict

Transfer With Booking Conflict Requiring New Seat Selection

Transfer With Booking Conflict Requiring New Seat Type Selection

To make a performance a) accessible via Box Office only or b)inactive on Box Office and on the Web:

In the Desktop Administration Module > Production > Edit Productions > Select the Production > Click on the Performances Button

Under the Access Dropdown on the performance select either Inactive to stop sales on Box Office and the Web for that performance OR Box Office to allow sales on Box Office only, removing the tickets off-sale for that performance only on the web. 

Don't forget to save:

Related Articles:

How to use the Cancelled/Postponed Production Converter (Booking Converter) Tool

Introducing the Cancelled/Postponed Production Converter