When a system is inherently toxic, there is no way to make it not-toxic short of coming up with a different substance. We can't make nuclear waste pure just because we wish it were. This is a very similar situation to this particular type of game. When it's a team game, and you rely on people who aren't doing their job, and you're separated by distance, it brings out the worst in you.
The problem isn't chat. If it were, it would be solved already, for the reasons you point out. The problem is the people. When someone doesn't like someone else, they will find a way to ruin their game while bypassing your censors. And yes, you can ban some of them, but not most of them, and not when most of the community acts like this. Which they do. Which you can't understand unless you go play the game for a bit.