This is pretty quick and easy to make if the requirement is no fluff is allowed to form at all rather than a particular puff, so I thought it might be easier for your learning to make it and let you look at the resulting script rather than try to explain it. I'm about to go out now but ask if you've got any questions and I'll happily answer them later.
Step 1: Copy the following to your clipboard:
Set var "ChallengeFail" = 0
If ...
Wait for clean room
Go to Skip to End
If End
Wait for clean room
If ...
Wait until var "ChallengeFail" = 0
Challenge completed Say No to Fluff
If End
Label Skip to End
Step 2: Create a character somewhere in the room and press Ctrl+Shift+B to paste the above code into his code box.
Step 3: Copy the following code:
Wait for event Puff merged into fluff
Set var "ChallengeFail" = 1
Step 4: Copy it into a DIFFERENT SECOND character by using Ctrl+Shift+B.
This challenge will trigger the turn AFTER you clear the room if you are successful at the challenge.