Sorry for the late reply. I had to learn some things about how this works!
It appears that when you open a .BloomPack which has reader settings, it stores a copy of those settings here:
Bloom will then use those settings when you create a reader for that language.
So if you’re sure you want to get rid of your settings for a given language, you could delete those two files. (You might want to move them somewhere instead so you have a backup.)
As far as deleting the books, there is no way to do a mass deletion of every book which was created using a given decodable reader template.