How can I stop my Servo rotating?
Unlike a normal servo motor where the argument of the Arduino's servo.write(angle) method represents a position, a continuous servo motor uses the argument for speed and direction.  Most continuous servo motors interpret stop to be approximately 90 degrees.  Full, say, reverse as 0 degrees.  And full, say, forward as 180 degrees.  As stated in the above link:


  On a continuous rotation servo, this will set the speed of the servo
  (with 0 being full-speed in one direction, 180 being full speed in the
  other, and a value near 90 being no movement).


Also of interest, most continuous servo motors have an adjustment to set the "90 degree position" (full stop).  That is to say, servo motor control signals are analog, not digital!  It follows that the processor's interpretation of the "90 degree position" may not be the same as the servos' interpretation of the "90 degree position".  Hence the ability to adjust the servos' interpretation.

This picture from the parallax servo tutorials web site shows how this is done: