Hi, I'm assuming your not using Exchange as Outlook is showing a "Personal Folder" and not a Exchange users folder
I'm not sure why it's not letting you add it to the block list through Outlook, But you could try this:
Change your Outlook View to have a Reading Pane, So you can see the contents of the email without opening it,
To do this, click on "View" - Reading Pane - Right
Then highlight the email, You should see a preview of the message on the right hand side,
More importantly - At the top of the email, you should be able to see the display name / email address it has come from,
Once you have discovered the senders email address, ( which is likely to be a relay through someone else's address).
You can manually add it to you're block list, OR create a rule to auto delete when you receive mail from that particular email address,
Hope this helps.