It is possible to disable any channel which will help reducing the total power consumption, but this will not stop the data of those channels being output even if the pins are left open or connected to ground.
What it can be done, is to set up the DCLK in such a way that there is no time to shift out the data of last channel/channels.
For example, if DOUT_FORMAT=1 (all channel are output through DOUT0) and channel 8 is not used, the data communication can be interrupted after the data of channel 7 is clock out