Ulduar is bugged , demolishers wont dissapear even when you kill first boss so you can use them to kill another bosses

I will have to agree with you on this one.
The instance is bugged and the demolishers won't disappear, but only if you're a hacker and teleport and fly all over the place...
Sadly, that's why you got perma banned.

Now on a more serious note, the instance is blizzlike and the despawn time of the demolishers after you kill Leviathan, is 5 minutes.

But thanks for the intent, it's sad you had to leave us so soon.