TeamSpeak 3 has this feature called whispering which allows a person to talk with some1 that is not in his channel .. think of it like a "private chat" to some1..

This can cause annoyance very fast therefor i suggest you to consider the case of blocking/denying it.

"How to do that?"

Go to : Settings -> Options

And then go to "Whisper" from the left side -> set "Deny everyone" -> click ok or apply to disable to be whispered by others.

"Can't this just be disabled it from the TS3 Server?"

No, it's not possible because the TS3 developers didn't made a way to disable this feature from the server. It can be done only from the client like i said above.

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^{This post is related to "popel" case which happen on 26.06.2011}
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epsislow