Flash :: Using Multiple Timer To Show / Hide Enemies?

i have a shooting game that works with some sort of rounds, for instance, this round is gonna show 4 enemies on the screen that the user has to shoot. I show the 4 enemies at a half seconds interval of each one so it doesnt all appear on the same time. Using something like:

enemiesShowTimer = new Timer(0.5 * 1000, 1);
enemiesShowTimer.addEventListener(TimerEvent.TIMER, showEnemyAtTime);
enemiesShowTimer.start();

The player has 2 seconds to kill each enemy after they appear, so i also use this:

enemiesCleanTimer = new Timer(roundConfig.getSecondsPerEnemy() * 1000, 1);
enemiesCleanTimer.addEventListener(TimerEvent.TIMER, cleanEnemies);
enemiesCleanTimer.start();

The problem is, after the player dies i change scene and if i still have like 3 alive, enemies, the threads will try to run the methods and it will crash. I'm using always the same variable to the start the timer whenever i need it. How can i solve this? Will i have to store each "thread" (timer) on a list and then stop each one separately? Because the way it is, the other threads are in some sort of "limbo" and i cant stop then, just the last one.