How about implementing an Peer-2-Peer based voice chat system, allowing users to be able to talk to eachother simply through the use of a microphone. Baseing it on a P2P system would make it so that there would be no additional burden placed on the server and a client would have the option to Enable/Disable voice chat at logon. You would be able oto block other users the same wasy you do for text chat.
Just an idea

EDIT: I just saw the other post for voice chat, sorries

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