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)