I had to squint at my screen when reading this to try and understand the logic behind it, and I still didn't see it... but I could be missing something.
Having (or not having) a clan tag doesn't prevent people from trying to kill one another or affiliating with other playing groups. With that in mind, it becomes clear that all this idea would really do is prevent people from leaving a clan when their relations with one or more other playing groups degenerate into war, trapping them in a PvP style situation that they may not actually want to be in.
That doesn't sound very fun, especially if the person(s) in your playing group are the aggressors and you don't agree with the way they are behaving.