Block Email Senders
You can block email from specific senders or domains in OWA webmail. I believe this will work for you even if you use other email clients, as the filtering happens on the email server.
In short:
Click the little gear icon in the upper right corner of the webpage and select Options.
Select "block or allow" on the left-side.
Add the email address you want to block under "blocked senders".
For more information, see the official Microsoft documentation at: https://support.microsoft.com/en-us/office/block-senders-or-unblock-senders-in-outlook-on-the-web-9bf812d4-6995-4d19-901a-76d6e26939b0#bkmk_classic