What is the recommended way to handle unused channels on this part?
The inputs of unused channels should be biased to prevent an indeterminate output state.
For the unused channels, I'd recommend connecting the DIN+ input to the 1.8V VDD1 power supply rail, and the DIN- input to the GND1 rail. This will set the channel in a defined logic 1 output state.
The DOUT+ and DOUT- outputs of the unused channels can be left floating. I'd recommend not terminating these channels, to save on power consumption,