Hey Laura,
I have the impression that airbnb uses a per-host rating system. Meaning that besides individual review score for each house, every listing is affected by the host's score.
By declining reservations you hurt your "score" as a host, not the listing's score. So in order to undo the harm, I would say you will need to delete your airbnb account and make a new one.
This is my understanding regarding the airbnb rating system.
On the other hand, I would recommend just sticking to your current situation and trying to undo the bad impression of the cancelations by being an exceptional host. If you are willing to sacrifice the time to redo your account and your listings it will probably be easier and more "fair-play" to do the extra effort and improve your overall score.
Just my 2¢
Harry
Baby-superhost of Greece