How to power off a dual supply op amp?
What you need is to is to either make the voltage difference between two nodes zero or make open circuiting so that they can not reference each other in order to actually power off. 

Cutting one would be open circuiting the internal circuit therefore powering off the op-amp.

As for the "best way": the rest of the circuit is of importance here, since probably powering the op-amp off change other stuff.