I actually don't know what backend enchant uses. The enchant.ordering file shows that myspell has priority over aspell. I was getting frustrated looking for things about myspell, however, and it seemed like Thunderbird might use myspell, and Thunderbird was seeing "Activites" as misspelled. I ended up just searching for "Activites" with this command:
It found ~/.config/enchant/en_US.dic, which contains a short list of the words in my personal dictionary. I deleted "Activites", and now both Bluefish and Abiword show it as misspelled.
find . -maxdepth 5 -print0 -type f | xargs -0 grep "Activites"