Exchange server 5.5

After deleteing a user from both User manager and exchange, the server still tries to deliver the message to the user. It does not reply to the sender that the account is no longer vald. Also, bulk mailers tries to use that account to spam mail and clogs up the server. I need and have to no avail researched a filtering agent in Exchange to let the sender know that the mail box is undeliverable, and that it would notify bulk mailers that this account is no longer availble. Any help would be Great.. Thanks