Why can't I remove a group member with a non-zero balance?
However, if you still want to remove someone who is not settled up, you can always transfer that person's debt outside of the group, i.e. settle up in the group and then add a new expense outside the group for the amount they still owe you. That will allow you to remove the person from your group, while still keeping a record of the $XX.XX they need to pay you.