How should I design it if there is an unused channel in multi channel circuit of Comparator?

Comparator

The designer should treats comfortable for not having an influence to other circuit in an appropriate if it occurs unused channel on your application. It needs to treat all unused circuits to be not transition.
The inverted input terminal and the un-inverted terminal should connect the most high voltage and lowest voltage with each terminal in common mode input range of the Comparator. (Fig1.a ) It makes the output terminal in the open state.
For products such as NJM2901 where the common mode input range is same as a supply voltage range can directly connect each terminal to a supply terminal and GND terminal without a resistance. (Fig1.b)

However, if the NJM319 comparator, which is guaranteed to have a maximum differential input voltage of ±5V, or a comparator with an NPN input stage is operated with Fig. 1.b, it may be destroyed by exceeding the absolute maximum rating. 

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  Fig1.a. expedient to a product for such as NJM2901

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  Fig1.b

Note: See a section entitled, "a way to treat the unused Op amp" on using an Op amp.