Original Source: https://help.yahoo.com/kb/SLN16025.html

Make sure you're only getting emails from people you want. Anonymously block up to 1000 email addresses in your Yahoo Mail settings.

Block an email address

Mouse over the Settings menu icon

Click Blocked Addresses.

Enter an email address in the "Add an address" field.

Click Block.

Unblock an email address

Mouse over the Settings menu icon

Click Blocked Addresses.

Click an email address in the list to select it.

Click Remove.

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