thank you! good to know i was in the wrong section. i've unfortunately tried to make the same rule in the metadata download section instead of adding books, and it's still not working.
Preferences --> Metadata download --> Create rules to transform tags/authors/publishers --> tags --> add rule --> Remove the tag if it contains: FanFiction
and sadly, when trying to download something new from ao3, it is still tagging everything with FanFiction. i've also tried matches pattern and if it is one of as well, just in case "contains" is not the right thing to select here. no luck.
i also tried another tag, just to see if it was something particular about some plugin like fanficfare or something interfering with the rule system. I tried all the same stuff above, but with the word Angst instead. picked a random fic from my ao3 bookmarks that contains Angst in the tags that i dont already have downloaded, downloaded it, and both FanFiction and Angst are still listed in the tags. not sure what i'm still doing wrong.