Nickname: Remove a users ability to get streams on the server.
Channel: Remove a channel's stream. If no stream is currently active this command shoud be used as it will tell the java client that there are no streams for the channel and in some themes the video window will not appear. In other themes an away message might be displayed. This all depends on the specific setup of your streaming server.
Example:
/stream deauth Rick
/stream deauth #lobby
IRCops can always use this command. Channel operators can use this command if the channel is stream enabled (via an 'as chan' command).