This is a very important option for multiplayer scenarios, because it often removes the need to actually spot enemies with your eyes.
It's also very immersion breaking. A common situation is:
In-game Radio: "Enemy, Soldier, Bearing 210, 500 meters"
Teamspeak: "Oh, err, huh, my guy spotted something, should be there and there... I cant see it!"
The radio and group chat messages are distracting and unnecessary in MP.
I don't play PVP myself, but in that kind of environment it's probably even worse. I would consider it cheating.
If you search for related terms you will find countless threads asking how to disable this, dating back to Arma (1).
http://forums.bistudio.com/showthread.php?155425-Is-it-possible-to-disable-the-automatic-detection-of-enemys-in-multiplayer
http://forums.bistudio.com/showthread.php?176795-How-to-make-units-SHUT-UP
http://forums.bistudio.com/showthread.php?164833-Disabling-Automatic-AI-Contact-Reports
http://forums.bistudio.com/showthread.php?71190-Disable-AI-contact-reporting-on-DeDi-Server
http://forums.bistudio.com/showthread.php?133434-Auto-Report-problem
http://forums.bistudio.com/showthread.php?155470-AI-Reporting-contact
http://forums.bistudio.com/showthread.php?152981-Auto-Reporting
http://forums.bistudio.com/showthread.php?111969-Silencing-the-player-AI
http://forums.bistudio.com/showthread.php?67252-How-can-I-make-my-player-shut-up
http://forums.bistudio.com/showthread.php?163684-How-to-Disable-players-voice
http://forums.bistudio.com/showthread.php?155728-How-to-forbid-enemy-report-on-radio
http://forums.bistudio.com/showthread.php?155728-How-to-forbid-enemy-report-on-radio
http://forums.bistudio.com/showthread.php?163684-How-to-Disable-players-voice
And many many more...
There is even a mod trying to address this issue:
http://forums.bistudio.com/showthread.php?147952-MRB-Voice-Stop
Also available for ArmA 2.
I tried all kinds of workarounds using enableSenteces, enableRadio, disableChannels etc. But nothing is satisfactory.