Hi, and thank you for asking your question.
The only way to definitively make sure that none of your emails are received by a specific recipient, is if they have blocked your email address at their end itself. However, in order to make sure that you do not accidentally send an email to that recipient, you may remove them from your list of contacts, if already added. To do this:
Open up Yahoo Mail.
Click on the contacts icon at the top-left (second icon, next to the Mail icon).
In your list of contacts, check mark the contact you no longer want to send emails to, and then click "Delete" at the top.
If you need any further assistance with this issue, please be sure to reply back using the box below.
Otherwise, if you could kindly click on (and submit) one of the stars at the top of this page to rate my service, I'd greatly appreciate it. Thank you.