Is the person in your Contact List? If so, you'll need to delete their name from your Contact List first, and then add them to your Ignore list (so messages from them won't get through to you). Here's how:

1. First, make a note of their Yahoo! ID. You'll see it in the "Contact Card" that appears when you move your pointer over their name in your Contact List. (You'll need this so you can add it to your "Ignore List" in step 8 below.)
2. Once you've noted their ID, right-click on their name in your Contact List, and select Delete.
3. Click the Messenger menu.
4. Select Preferences.
5. Click Ignore List.
6. Select the option labeled "Ignore only the people below."
7. Click Add.
8. Enter the Yahoo! ID of the contact you want to ignore.
9. Click Ignore.
10. Click OK at the bottom of the window.

You can ignore up to 100 contacts.

To ignore everyone who is not on your Messenger List:

1. Click the Messenger menu and select Preferences.
2. Click the "Ignore List" category.
3. Select the option labeled "Ignore anyone who is not on my Messenger List."
4. Click OK at the bottom of the window.

When you ignore a contact, you always appear offline to the contact. They cannot view your status or send you an IM.

Please note: To remove a contact from your Ignore List, select the ID and click Remove.