Can't be done - Not the ads that looks like an email message
From Yahoo -
Remove or hide ads in Yahoo Mail
You can temporarily hide the advertisement appearing on the right side of free Yahoo Mail accounts or permanently remove all graphical ads by upgrading to Yahoo Mail Plus. Text ads and ads in other versions of Yahoo Mail can't be hidden.
Temporarily hiding ads in free Yahoo Mail accounts
Sign in to Yahoo Mail.
Click the narrow gray strip to the left of the advertisement on the right.
The ad is hidden from view.
A new ad will appear if you refresh the page or visit another Yahoo Mail page.
Ads that can't be hidden
It's not possible to hide:
Ads that appear in Yahoo Mail on mobile and Basic mail versions.
The text ads that appear at the top of your Yahoo Mail account's folders.
text ad with red box around it at top of email list
Permanently removing ads from Yahoo Mail
Yahoo Mail is made available in part by the advertising you view while using our site.
If you would like to remove all graphical ads from your mailbox, we encourage you to sign up for Yahoo Mail Plus, if it is available in your region.
- Yahoo Mail Plus accounts will not have ads when viewed on mobile browsers.