How to disable a single op-amp in dual packaging?

A unity gain buffer is what this article also suggests. Quote:

The best thing to do with a surplus op-amp it is to use it. There are lots of places in an analog circuit where a buffer amplifier may improve performance - and a unity gain buffer uses no extra components.

Grounding the non-inverting input is fine if you have a dual power supply. If you use a single supply you better set the input to Vcc/2 with a resistor divider:

(from the same article)

After reading many opinions here and also here and here, as well as Maxim AN1957 and TI SBOA204A application notes, I've decided to combine all of that into one simple to follow cheat sheet.

Notes:

• The op amp inputs must be actively held within the input common-mode voltage range of the device.

Devices with dual supply usually have ground well within allowed input range, and so for this reason their non-inverting inputs can be connected directly to the ground.

With single supply rail the mid-supply is also usually within common-mode voltage range, so selecting two equivalent resistors ensures that the input voltage will be within this range.

• The op amp output must be set within the output voltage swing range of the device.

As output range usually includes mid-supply for both single and dual rails, the simplest solution for unity-gain stable devices is to tie output to inverting input in voltage follower configuration.

For non-unity gain devices this means additional resistors needed to implement basic non-inverting configuration and keep the amplifier in its linear operating range. For single supply this could also require tweaking the input.

• As any cheat sheet the above should work in most cases but of course variations are possible (note the heavy use of "usually" in the above).

For example some reference voltage elsewhere in the circuit that is within the input common-mode voltage range can be used on non-inverting inputs to eliminate two resistors.

The suggested schematics use 2x gain for non-unity circuits. In some cases this might not be enough for stability, so the gain should be adjusted according to op amp specification.