In Yahoo!mail Beta, how do I blacklist a sender who keeps showing up in my spam folder?

If they are in your spam/bulk folder then they are going where they are suppose to go.

Blocked addresses are email addresses that you do not want to receive mail from. Incoming mail from these addresses will be automatically disposed of, without bouncing back to the sender.

When you remove an address from your Blocked Addresses list, you will once again be able to receive mail from that address.

If you receive a message from a sender and want to block that sender, follow the directions below:

From your inbox, check the box next to the message.
Click the “Spam” button at the top of the page.
The message will be marked as spam and the sender’s email address will be added to your Block List.
If this option is not working, make sure you have enabled this feature in your Spam Protection settings. To check the settings, click “Mail Options” and then on “Spam Protection.” On the Spam Protection page, make sure “Add the sender’s email address to my blocked addresses list” is checked and then click the “Save Changes” button.

Click the “Block Address” link next to the sender’s email address at the top of the message.
You will be returned to the original message with a notice at the top of the screen that the address is now blocked.
To manually add an address to the list of blocked addresses:

Click the “Mail Options” link on the top right navigation bar of your AT&T Yahoo! Mail page.
Click the “Block Addresses” link in the middle of the page, under the “Management” column.
Enter the new address in the “Enter email address to block” field, and click “Block Address.”
The address will appear on your list of blocked addresses.
To remove (“unblock”) an address:

From the Blocked Addresses list, check the box to the left of the address you wish to remove.
Click the “Delete Checked” button at the bottom of the Blocked Addresses list.