Primary Method:
1. Open Outlook.
2. Click on the unwanted message.
3. On the ribbon, the last option on the right should be "Barracuda Networks". In this box you will see "Mark as Spam".
4. Click on "Mark as Spam" for any unwanted email. 

Secondary Method:
1. Open Outlook. 
2. Right-mouse click on the email wish to prevent. 
3. Hover over "Junk"
4. Click "Block Sender"