def remove(self, elem):
        """Remove a requirement.

        :param elem: a string or :class:`Requirement` instance.
        """
        if isinstance(elem, text_type):
            for requirement in self:
                if requirement.package == elem:
                    return super(Requirements, self).remove(requirement)
        return super(Requirements, self).remove(elem)
def ban_user(user_id=None):
    if not Permission(CanBanUser, identity=current_user):
        flash(_("You do not have the permissions to ban this user."), "danger")
        return redirect(url_for("management.overview"))

    # ajax request
    if request.is_xhr:
        ids = request.get_json()["ids"]

        data = []
        users = User.query.filter(User.id.in_(ids)).all()
        for user in users:
            # don't let a user ban himself and do not allow a moderator to ban
            # a admin user
            if (
                current_user.id == user.id or
                Permission(IsAdmin, identity=user) and
                Permission(Not(IsAdmin), current_user)
            ):
                continue

            elif user.ban():
                data.append({
                    "id": user.id,
                    "type": "ban",
                    "reverse": "unban",
                    "reverse_name": _("Unban"),
                    "reverse_url": url_for("management.unban_user",
                                           user_id=user.id)
                })

        return jsonify(
            message="{} users banned.".format(len(data)),
            category="success",
            data=data,
            status=200
        )

    user = User.query.filter_by(id=user_id).first_or_404()

    # Do not allow moderators to ban admins
    if Permission(IsAdmin, identity=user) and \
       Permission(Not(IsAdmin), identity=current_user):

        flash(_("A moderator cannot ban an admin user."), "danger")
        return redirect(url_for("management.overview"))

    if not current_user.id == user.id and user.ban():
        flash(_("User is now banned."), "success")
    else:
        flash(_("Could not ban user."), "danger")

    return redirect(url_for("management.banned_users"))