As you continue to mark certain emails as junk,Outlook 'learns' over time which emails to filter, but to ensure that you never receive email from a particular address/sender, you can block them.
1. Right click on the email and scroll to "Junk..."
2. Select Block Sender.
3. Alternatively, you can click on "Junk E-mail Options..." and select "Blocked Senders" from the ribbon.
4. Click "Add..."
5. Add the email addresses/domain names you would like to block and click OK to finish.