I think this make it a lot more immersive and realistic but there is no possible way to implement this. Even if you force direct chat on all the time, people will just specify a different default input for DayZ to use. It will have the opposite effect, people will actually disable their mics for dayz so they can continue to have private chat on TS, Mumble and Skype. DayZ/Arma 2 doesn't support stereo inputs either so some people like myself could connect their mic to the other channel (arma engine only takes one channel) and programs like mumble mixes stereo inputs to mono, so everyone on mumble could still hear them. Not allowing DayZ to run while these third party applications are running won't work either. Apart from obvious software workarounds, People could use another device. Most people have access to smart phones, tablets. Mic's with a built in mute button are cheap that some people could get for use with dayz. I think people who want this experience only have the option to play on a private server where this is a rule and you only allow trustworthy players into the community. You will obviously get some breaking the rule, but it will cut it down a lot.