how to Spawn prefab infront off player and destroy it when hr passed it

so im making this endless runner game and want to ranomdly spawn objetcts on a road 100 - 150 units infront off the player and destroy the spawned prefab when he passed it.

heres the code i have for that

float preference =10 ;

    if(spieler.transform.position.z > preference)
     {
         for (int i =0; i < enemyAmount; i++)
         {
             spawnPointx = Random.Range(-12.346f, 6.73f);
             spawnPointz = Random.Range(spieler.transform.position.z + 200, spieler.transform.position.z + 400);
             Instantiate(prefab, new Vector3(spawnPointx, 3, spawnPointz), Quaternion.identity);
         }
         preference = preference + 150;
     }