...
Outlook Web App (OWA)
Sign into Outlook Web App with your Login credentials.
Click the Settings gear in the top-right corner of the screen.
Click Viewall Outlook settings at the bottom of the Settings menu.
Click Mail in left-hand menu.
Select Junk email. There will be options for blocking senders, setting safe senders, etc.
...
If you attempt to mark a sender as being Junk, but continue to see messages from that sender in your Inbox area, we suggest managing your Junk email list through Outlook Web App.