There must have been a moment at the beginning, where we could have said no. Somehow we missed it. Well, we'll know better next time.
I am fairly certain that they have to remove themselves. This is one of the main drawbacks of using BGG's guild feature (which has many advantages as well, of course).
I recommend contacting them via geekmail and asking them to remove themselves from the guild.
Or if they're MIA, incommunicado and haven't logged in in years, I'm sure an admin could do it.