If you need to disable the auto-assignment function and hence have all new tickets assigned to the default agent (set in the plugin general settings), you can add this constant in your theme’s
define( 'WPAS_DISABLE_AUTO_ASSIGN', true );
Note that this will only disable auto-assignment in the core WordPress plugin. It will NOT disable auto-assignment in the Smart Agent extensions/add-on. To disable it there, simply deactivate the extension in the WordPress Plugins screen.