Is there a library, class or method that an addon can access to be able to delete members?
Yup, just load the Member model:
ee()->load->model('member_model'); ee()->member_model->delete_member($member_id, $heir_id);
$member_id can be a single member, or an array of ids.
$heir_id is the
member_id that should take authorship of any existing entries belonging to the member being deleted.