How to Block a Sender in Yahoo! IM
#

Open your Yahoo! Messenger program and sign in. Delete the Yahoo! ID of the person you would like to ignore from your contact list by right-clicking on the name and navigating to "Delete." Click "OK."
#
Step 2

Go to the "Messenger" menu and click "Preferences." Highlight "Ignore List" under the heading "Category."
#
Step 3

Select "Ignore Only the People Below" and click "Add."
#
Step 4

Type the Yahoo! ID of the person you would like to ignore in the **** box. Make sure "Yahoo! Messenger" is selected in the pull-down menu on the right. Click "Ignore" and then click "OK" to close the ******** You will now appear offline to that person as long as he is on your ignore list.
#
Step 5

Click on "Ignore Anyone Who Is Not in My Messenger List" in the ignore list preferences screen and hit "OK" to ignore anyone who is not in your contacts list. You will appear offline to everyone except those in your contacts list

thanks sir Zalvaje for this post..