Problem with my Coroutine

Hi i am creating an asteroid game and for my asteroids i used a Coroutine. Now i am trying to create boss fight after i destroyed 100 asteroids. How can i do that. I tried to stop the loop but it wont turn back again. Also if i set up do to something when is bigger then 100 asteroids the other setups wont stop. Sorry for my English and also for my post i am new to this domain. Thank you very much.

public class RandomHazzards : MonoBehaviour {

 public GameObject Hazzard;
 public GameObject Hazzard2;
 public GameObject Hazzard3;
 public GameObject Hazzard4;
 public GameObject Hazzard5;
 public GameObject Hazzard6;
 public GameObject Hazzard7;
 
   
 public GameObject Spawner;

 public Vector3 SpawnValues;
 public Vector3 SpawnValues2;
 public Vector3 SpawnValues3;
 public Vector3 SpawnValues4;
 public Vector3 SpawnValues5;
 public Vector3 SpawnValues6;
 public Vector3 SpawnValues7;

 public int HazzardCount;


 public float spawnwait;
 public float StartWait;
 public float WaveWait;
 public float IncWave;
 public float Waveplus = 0.2f;


 public int DestroyedAsteroids;



 void Start()
 {

     StartCoroutine (SpawnWaves ());
 

 }
 void Update()
 {
     GameObject theasteroid = GameObject.Find("GameController");
     GameController gamecont = theasteroid.GetComponent<GameController>();
     if (gamecont == null)
     {
         //   Debug.Log("Game COntroller not found");
     }
     if (gamecont != null)
     {
         // Debug.Log("Itworks");
         DestroyedAsteroids = gamecont.asteroidskilled;
     }

     IncWave = WaveWait - Waveplus *Time.deltaTime ; 

     if (IncWave <= 0.5f) {
         IncWave = WaveWait;
     }
 
 }

 

IEnumerator SpawnWaves() {

    yield return new WaitForSeconds(StartWait);

     while (true) {


         for (int i = 0; i < HazzardCount; i++) {

                 Vector3 spawnPosition = new Vector3 (Random.Range (SpawnValues.x, -SpawnValues.x), SpawnValues.y, SpawnValues.z);
                 Vector3 spawnPosition2 = new Vector3 (Random.Range (SpawnValues2.x, -SpawnValues2.x), SpawnValues2.y, SpawnValues2.z);
                 Vector3 spawnPosition3 = new Vector3 (Random.Range (SpawnValues3.x, -SpawnValues3.x), SpawnValues3.y, SpawnValues3.z);
                 Vector3 spawnPosition4 = new Vector3 (Random.Range (SpawnValues4.x, -SpawnValues4.x), SpawnValues4.y, SpawnValues4.z);
                 Vector3 spawnPosition5 = new Vector3 (Random.Range (SpawnValues5.x, -SpawnValues5.x), SpawnValues5.y, SpawnValues5.z);
                 Vector3 spawnPosition6 = new Vector3 (Random.Range (-SpawnValues6.x, SpawnValues6.x), SpawnValues6.y, SpawnValues6.z);
                 Vector3 spawnPosition7 = new Vector3 (Random.Range (-SpawnValues7.x, SpawnValues7.x), SpawnValues7.y, SpawnValues7.z);
         
                 Quaternion SpawnRotation = Quaternion.identity;


                 if (DestroyedAsteroids >= 0) {
                     Instantiate (Hazzard, spawnPosition, SpawnRotation);
                     Instantiate (Hazzard2, spawnPosition2, SpawnRotation);
                     Instantiate (Hazzard3, spawnPosition3, SpawnRotation);
                     Instantiate (Hazzard4, spawnPosition4, SpawnRotation);
                     Instantiate (Hazzard5, spawnPosition5, SpawnRotation);
                 }

                 if (DestroyedAsteroids >= 20) {


                     Instantiate (Hazzard6, spawnPosition6, SpawnRotation);

                 }

                 if (DestroyedAsteroids >= 40) {
                     Instantiate (Hazzard7, spawnPosition7, SpawnRotation);
                 }
                 if (DestroyedAsteroids >= 70) {
                     Instantiate (Hazzard6, spawnPosition6, SpawnRotation);
                     Instantiate (Hazzard7, spawnPosition7, SpawnRotation);
                 }
                 if (DestroyedAsteroids >= 90) {
                     Instantiate (Hazzard7, spawnPosition7, SpawnRotation);

                 }

                 yield return new WaitForSeconds (IncWave);

             }
         }
     }
 }