Update: I was also trying to remove another person from the role of Online Trainer.
I discovered that he had an "unsupported role" as Online Trainer at the site level and removed this.
However, he still showed up in the list of Online Trainers in the top level category, so I checked his user profile > assigned roles and found the he still had the Online Trainer role in a subordinate / child category.
After I removed this, his name disappeared in the list of Online Trainers in the higher-level parent category.
This seems to be a bizarre case of "reverse inheritance" of roles. Is this a bug or a feature?