When it gets to the point where the board of health is likely to condemn it, your tactic may need to change. In the meantime let it be. If they can t find their clothes, they might not be able to go out. When they lose their homework in the rubble, they face the natural consequences of their actions. Don t allow them to take anything to eat or drink to their rooms.
Dirt is one thing, ants another.
If it reaches the stage where it is really unhealthy, literally condemn it and force them to live out of a wash basket and sleep on the sofa. Keep advising them to clean their room at this point and each time they do not comply, remove a prized possession from the condemned area. If they still haven t cleaned it when the room is completely empty, they need to earn back the stuff. Top on the list as a way to earn possessions back is to houseclean that condemned area. This may earn them back the privilege to sleep in there again.