Please use this thread going forward: [BDK] Dust bunnies spawn in inaccessible places | EA Forums - 11871191
There a couple of ways to deal with the issue you ran into.
The first way is to prevent dust from building up on an inaccessible room. I use the lowest level basement on all my residential lots as a storage room where I stick stuff like the Christmas tree. There are no stairs or ladders to it. When I first got Bust the Dust I noticed that dust and the dust bunnies started showing up there. Not a big deal until I found that the dust piles and dust bunnies blocked me putting stuff in the basement. I never got any of the Filth Fiends though. And I didn't know they will set fires until reading your post.
Anyway, I have found a way to keep that basement clean so I don't get the dust piles and bunnies. But you have to have the Cat and Dogs Expansion pack. That pack has two robot vacuums. So you get one, do all the upgrades on the robot and the base (they are really 2 separate objects and each part has to be upgraded separately), set it to run on auto every six hours and put it in your room. I put one on every floor of my houses now and don't have any dust buildup at all. They do break on occasion so you do need to check that they are still running every now and then. But my house now stays pristine, including the inaccessible basement.
But you say you only have the Bust the Dust pack so that might not be an option for you so you will need to get a sim into the space to clean every so often. There are two ways to do that and they allow you to teleport a sim there. If you had Spa Day, there is a teleport ability once you level the Wellness skill up to level 7 or 8. So that might not be an option for you either since you don't say you have that pack.
The other way is with a cheat. Turn testing cheats on. Then right click on a spot in the room where you want to go. You should see a teleport option. Use that to get a sim into the room to clean (have to have a vacuum in the sim's inventory) or to put out the fire.
Hope this helps.
