I posted some messages in public Yahoo groups that I want to have erased. This is very important for me.?

You can delete those messages, but you need to be logged in as the same member of the Yahoo Group that the messages were posted under. When you are a member of the Group under the same login name from which you sent the mesasage(s) in question, go to the Group's web page and click on Messages on the left-hand side. You can then use the Search box to search for your messages. When you've found the message you want to erase, click on its Subject line to bring up the message. If the message was sent by the same login name you're signed in under, then just above the content of the message will be links to Reply or Forward or Delete. Click on the Delete link. Once you've confirmed that you want to delete this message, it will be erased. If necessary, repeat the searching etc. until you've deleted all the messages in question.