Hi Danilo,
You can adjust the default privacy when a new blog post is created in Settings>General>Default Options>Default Privacy Selection>selectDesiredPrivacy
If you don't want all users(including yourself) to change the privacy setting of the post in the composer, you can hide the privacy field in the composer at Settings>General>Composer>Composer Post Options>Show Privacy Section>No
Alternatively, you can also configure the ACL of the related user groups so that only certain user groups can change the privacy in ACL>userGroup>Compose>Change Privacy When Composing Blog Post>No
Hope this helps.