How do I ban someone from my group if they've already left?
If someone has a comment in your group and then left, you can still ban them from your group. From their post in the group, you'll see a link called "options". Click on that, and you'll see that you can ban the person right at the comment itself - even if they are no longer with your group.
If there are no comments left in your group, you can contact email@example.com and ask one of the caretakers to do this for you.
When you email the caretakers you should take advantage of the opportunity and report the member you want to ban from your group.