Arduino Servo Free Rotation
When you attach a servo the Arduino starts generating the PWM waveform for its currently selected angle. When you write to the servo you just change the current angle and the waveform changes to match. That waveform is constantly being generated from the moment the servo is attached, and will always be generated.  That is how servos work.  They are not really designed to be back-driven by the load turning the cogs - it is the role of the servo to do the driving, not to be driven.  

Depending on the servo you might actually damage the mechanics by driving it backwards like that.

The only way you can stop the servo from being actively driven is to detach the pin from the servo system so that it doesn't think there is a servo there any more.