Hello, do you mean by 'notes' some message sent through the chatter?

If so, you you need to create a special subtype for such messages and re-define the function def _search of mail.message. Here you need to add some condition, e.g:

        group_ids = self.env.user.groups_id # the list of acess groups of this user

        group_user_id = self.env.ref('base.group_user') # employee group guarantees it is not portal

        if group_user_id not in [group.id for group in group_ids]:

            args = [('subtype_id', 'not in', subtypes_to_restrict)] + list(args)

        return super(mail_message, self)._search(args, offset=offset, limit=limit, order=order,

            count=count, access_rights_uid=access_rights_uid)

Where subtypes_to_restrict is the list of messages subtypes, portal users do not have a right for.

In order to avoid sending notifications to portal users, just do not subscribe them for those subtypes.

Besides, this paid app - https://apps.odoo.com/apps/modules/10.0/internal_thread/ - let you send internal messages from any object (so internal notes attached to any records disregarding subtypes). Such messages would not be sent to portal users, and those users would not have access to them through the interface.