I am using `company-mode` for completion purposes. But somehow, some package always draws in `auto-complete` as well, which collides with `company-mode`, since both offer a popup with completion candidates. I want to blacklist `auto-complete`, since I do not need it. How can I do that? I know there is a variable `package-pinned-package`, but I cannot see how to blacklist a package — maybe by using an invalid archive?