How to use "But" properly

Which of these two sentence form is more accurate and why:

1. Ramiro is anything but professional.
2. Ramiro is anything but not professional.

These both sentences conveys the same message about Ramiro being anything except professional. But which sentence should be used?

Ramiro is anything but professional. Ramiro is anything but not professional.

But, here, means except. In the first sentence, you are saying that Ramiro is not professional. (He is anything except professional). By using not in the second sentence you are reversing the meaning; so now you are saying that Ramiro is professional. (He is anything except not professional.)

Now consider

Ramiro is anything, but he's not professional.

This is equivalent in meaning to your first sentence.