just say to him double tap voip key, else you die. If he is using speakers, you know he won't survive very long with all those chats flooding in...

sure. thats the easy way. but he easily can turn it off, talk quickly to others and turn on again...
with my idea, you dont have to watch him permanently, you know...

You ask for restrictions for another soft. It is impossible and even if it possible not any dev have rights to turn off another programs on someone's PC.

Okay, they will forcely turn off Teamspeak/Skype/etc but I can easily take my laptop (another device or just telephone or just meet in one room) and speak with anyone I want.

i didnt said to turn of teamspeak/skype etc.
i said to force enable ingame voip. so they cant talk, unless we can hear it. ;)

you have to read all of the post. :)

Could be circumvented with any kind of Virtual Audio Cable program, so would be useful for about a week until someone posts on reddit how to get around it.

Its not possible to controll the person wo plays the game.

Your proposed feature can always be circumvented with a capacitor-based microphone (cheap) and additional (USB) soundcard. One could simply switch off the mic for ingame VoIP and still use the second one for TS. You want steam to check for a second device? People will use tablets/smartphones for TS.

in life its allways possible to cheat.
but the question is, how much money / time / work would I use to get what I want?
this solution can be circumvented. but not without investment. therefore probably the most players would just take it and would not buy some super special sound equip to hack the game...

well I see the problem in trying to prevent this.
I also would like to be only able to use ingame communication, like ingame voice and or walkie talkie. that would be very interesting and immersive... but yeah.... I don't see how you could prevent this...