Is there any way to disable spam filter completely?

In typical 'Microsoft' fashion, they have decided they know more about your needs than you do, so there is no way to disable the filters. 

What you need is a workaround.

Create a new rule that says If Sender contains: "a" OR "e" OR "i" OR "o" OR "u"  then move message to the inbox.

There's a good chance that any sender will contain a vowel. 

Unfortunately, you can't simply use "@" because the great and all-knowing system won't accept is as "valid text." 

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On second thought, forget it.  After extensive testing it seems the message rules only work about 20% of the time.  Any mail from a real message server is dumped in the junk bin regardless of contents.

Thanks Microsoft.  You've done it again.  Created a system that's both free and worthless.

For those of you that are using Office 365, the following document has steps to disable the Microsoft spam filter.  Skip down to page 6.  FYI, I have not tried this.  I am in research mode before I move a bunch of users over to Office 365.

McAfee Spam Filter with Office 365

I'm ditching outlook over this, even though I use WP. This is ridiculous! Only useless replies of http://answers.microsoft.com moderators top this. I'm currently investigating a cheap plan at https://www.fastmail.com/ and with the available alias options there I don't see myself sticking with outlook.com even as a one time junk email. I started recommending outlook.com to my friends to drag them off gmail but I'll no longer do so.

This is true for normal Spam. In our case we generate our own Spam by forwarding form a Website and partially Spoofing the senders eMail from a Textbox (so we can reply directly via eMail). But good you found a solution for most users, thank you!