Webmail (Outlook on the Web) and the Outlook desktop client have built-in tools to block junk/spam or any other unwanted messages. Here's how.
How to mark email as junk in Webmail
How to mark email as junk in the Outlook Desktop client
Select the message you want to mark as junk.
At the top of the screen, select Junk > Junk to report that individual message as junk and move it to your Junk folder.
If you want to completely block all messages from that sender, select Junk > Block (Sender)
Right click the message, then select Junk > Block Sender