...
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.
Outlook
...
for Mac
Click Junk.
From here, you can click Junk or Block Sender.
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.