BuddyPress.org

BP uses the WPMU user accounts. If you have deleted the user account in question, they won’t be able to log in. You could check the DB to confirm that.

Only other thing I can think of is if you are using caching and it has not refrenshed yet. Some BP widgets’ content is generated when it happens. E.g. The “recent activity” block – if you want to clear that one out, you’ll find it in a DB table.