The missiles don't "lock" on to anything. They assume a ballistic path calculated to intercept the incoming missile derived from our sensor network. The intercept payload "locks" on to the incoming payload when it is within range.
They can. What counts is how many missiles get through. Some will, some won't. That is what should prevent an enemy from even trying: he won't know which missiles will get through, so his military plans will be useless and what comes back will make his military plans irrelevant anyway. A further complication is that the missile defense would have to be neutralized before the main attack and that will provide enough time to make the necessary response and put an end to the threat. There will be some successful strikes; that is impossible to stop completely.
The Aegis BMD system with it's SM-3 missiles looks like a great system but probably doesn't have the range to take on the NK ICBM.
In fact the SM-2, SM-3 and PAC-3 systems all look like theater defense systems to me. Not clear that we would be close enough to the launch to take out the NK missile.
There is some good background information on the SM-3 and the Aegis system at this website
Here is a key paragraph:
The Aegis BMD weapon system went to sea September 30, 2004. Raytheon began delivering Standard SM-3 initial deployment missile rounds to the Missile Defense Agency (MDA) October 22, 2004. Five additional missiles were delivered to MDA in December 2004 to support missile deployment on destroyers and cruisers. Aegis BMD will protect not only the United States, but also US allies and US troops deployed around the globe against short to medium range ballistic missiles. SM-3 has been under testing since January 2002 intercepting targets in space four times.
Note that the system is for short and medium range ballistic missiles. I think the NK missile is in the long range class. Hoping I'm wrong about this. I would love to see us take out that SOB's latest toy.