MonoBehaviour.StopAllCoroutines  

// Starts the coroutine
StartCoroutine ("DoSomething");
// Cancels the coroutine immediately afterwards
function DoSomething () {
while (true) {
yield;
}
}
StopAllCoroutines();

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
IEnumerator DoSomething() {
while (true) {
yield return null;
}
}
void Awake() {
StartCoroutine("DoSomething");
StopAllCoroutines();
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def DoSomething() as IEnumerator:
while true:
yield

def Awake():
StartCoroutine('DoSomething')
StopAllCoroutines()