Is there any way I can turn off the 5V pin?
For energy-efficient switching (along with relatively high-current capability), I recommend using a Darlington transistor connected to one of the output pins (digital for on/off, analogue for variable). Then you would just call  pinMode(A0, OUTPUT); followed by digitalWrite(A0, HIGH); Here is a sketch I made demonstrating what I mean:


Note that a normal transistor would draw more current from the Arduino's pin, which could be bad for the Arduino, and which would waste energy.