I think you misread the double negative?
You're right / I wasn't clear.
The student loan confirmation is sent to spam every month. I mark it as not spam.
In their defense, the message is full of all caps words that look like a sentence formatted from a sql query with little attention paid to formatting plus it has all sorts of spam words (bank, payment confirmation, loan, etc).
That's because it's not spam. I'm pretty sure there's a blog post coming soon about the difference between spam and "email I don't want to receive". If you just don't want to hear from a particular email address, the way to stop it is to put an explicit discard rule for that address in your Rules screen.
Likely changes for next year include making a much easier UI option for "don't show me email from this address again", because that's a fairly common request for situations like this where you have a vexatious sender. I suspect it will be implemented as an addressbook group - the anti-whitelist!