Getting emails from a specified sender that you’d rather not to see? Well, there is a way you can stop those emails you consider as spam. However, if you don’t know how to stop spam emails Yahoo can restrict, you must set the filter for stopping spam email straight away.     

Block email addresses
Click the Settings icon. | click More Settings. .
Click Security and Privacy.
Under "Blocked addresses", click the Add button.
Enter the email address you want to block.
Click Save.

Greetings,
Peter