Yes, you certainly can use the unused gates.
I only grounded their inputs because you can't leave any inputs floating with a CMOS circuit.
Those are NAND gates so the logic is that only when both inputs are high is the output is low, otherwise the output is high.
If you want to prevent a signal from going through the gate then you keep one input low. This keeps the output high, regardless of what the the other input does.
I only grounded their inputs because you can't leave any inputs floating with a CMOS circuit.
Those are NAND gates so the logic is that only when both inputs are high is the output is low, otherwise the output is high.
If you want to prevent a signal from going through the gate then you keep one input low. This keeps the output high, regardless of what the the other input does.