When a news company is arriving, it's perfectly possibl to kill it if you're already strong - and have smaller planes with enough range to attack ALL lines at the same time. I insist on smaller planes. It's tough & costly, as you'll be plagued by slow rate of new planes(you need a lot to sink your opponent) and increasing slot costs.
It's usually not worth the hassle, and unneeded anyways. the opportunity cost is too strong. It's more intelligent to open a new base elsewhere with more potential. In GW3, I've seen 7 opponents settle in Glasgow, after being first there. 6 BK'd without help from me(or just at the end, when they were already dying, ). The seventh is still alive, but not on the same market, and anyways, 75% of my traffick is in other bases. S, who cares? I've got 1 opponent in Bari, too, who is much more annoying, but he's got 7 bases - that means 6 I cannot attack. I can't kill him, he can't kill me. I've been stupid enough to lead a price war against him, but quickly gave up. It only killed my income without hurting him enough.