Follow these steps to block emails from a sender you deem is spamming you...
This ONLY applies to Outlook Web Access (OWA) email, using your browser for emails on Office 365.
RIGHT-Click on the email in question, in the email list and you should see a menu pop up, like this:

Click on "Block", and you will get two options, like this:
Click on "Block sender"
The email will be removed and the sender's email address will be blocked.
Please remember that MANY spammers use randomly generated fake emails and blocking one may not stop others from the same spammer, as they vary the email address slightly each time. The only thing to do is continue to BLOCK them.