Basically if you are being ignored by someone you can still send them mail. Not sure if this is meant to be able to happen or hasnt been thought of. But if you ignore someone they can send nasty messages in the mail =P
-1 If they send nasty messages, screenshot it and report. That's illegal. IMO this isn't needed, and the ability to send mail is. Say I held an auction, and the winner was ignoring me. I couldn't send him what he won... it'll create more problems than it solves.