To block someone from sending you an instant message (from Outlook Express)
To block someone who is online
In the Contacts pane in Outlook Express, double-click the name of the contact you want to block.
In the instant message window, click Block.
As long as this person is in your Block list, you will not be notified of any messages this contact attempts to send, nor will he or she be able to see your online status.
To block someone who is not online
In Outlook Express, click the Tools menu, point to the name of your messenger service, and then click Options.
Click the Privacy tab.
In the My Allow list, click the name of the person you want to block, and then click Block.