When I try to set my Junk E-mail filter to "No Automatic Filtering", Outlook will switch it automatically back to Low.
How can I completely turn off the spam-filter?
This happens when you have an Outlook Hotmail Connector account configured in your mail profile.
Junk E-mail settings are being synchronized
Besides email, the Outlook Hotmail Connector also syncs your Junk E-mail settings as you have them configured it in your web based Outlook.com mailbox. However, the lowest possible setting for the Junk E-mail filter in the Outlook.com web interface is “Low”.
There is no way to turn it off completely in the web interface so you won’t be able to turn it off completely in Outlook anymore either as the “Low” settings would sync back.
Outlook 2013 and Exchange ActiveSync
When you use Outlook 2013, you’ll connect to your Outlook.com account via the Exchange ActiveSync protocol (EAS). In this case, your Junk E-mail settings are not being synchronized as EAS doesn’t support this.
This means that you can completely turn off the Junk E-mail filter in Outlook for your Outlook.com account.
Connecting to Outlook.com via POP3 or IMAP
A workaround would be to use POP3 or IMAP to connect to your Outlook.com account instead. You can then completely turn off Outlook’s Junk E-mail Filter. However, do note that Junk E-mail can still be filtered out by the Junk E-mail Filter of Outlook.com.
For an overview of the settings that you’ll need in order to configure POP3 or IMAP access to your Outlook.com mailbox see this guide.