How do I stop an Arduino sketch?

Take a look at the
Examples:Servo:Knob code in the IDE:

// Controlling a servo position using a potentiometer (variable resistor) 
// by Michal Rinott <http://people.interaction-ivrea.it/m.rinott> 

#include <Servo.h> 
 
Servo myservo;  // create servo object to control a servo 
 
int potpin = 0;  // analog pin used to connect the potentiometer
int val;    // variable to read the value from the analog pin 
 
void setup() 
{ 
  myservo.attach(9);  // attaches the servo on pin 9 to the servo object 
} 
 
void loop() 
{ 
  val = analogRead(potpin);            // reads the value of the potentiometer (value between 0 and 1023) 
  val = map(val, 0, 1023, 0, 179);     // scale it to use it with the servo (value between 0 and 180) 
  myservo.write(val);                  // sets the servo position according to the scaled value 
  delay(15);                           // waits for the servo to get there 
}

This reads a potentiometer, and moves the servo based on the reading.
Do the same but get the reading from your remote connection instead.

void loop(){
// check outside source, see if new position came in
if (new position came in && new position is within allowed limits){
// move the servo - say 5 degrees left or 5 degrees right depending on the command  - all depends on the protocol you set up.
}
else {
// do not move servo
}
} // end loop