The answers are difficult but one of them is to stop the nutjob in NK from getting a nuc capable ICBM and selling it to Iran. Too many people think NK is funny and an object of ridicule. (When I expressed concern on TOS I was told to lighten up and relax) They won't be laughing when NK or Iran nukes our western shore or Tel Aviv.
If you have ever been in Seoul the problem with any military action directed at the nutjob to the North quickly becomes apparent. Even a crude nuclear device detonated in that terrain by the North in response to a US military strike would cause an enormous loss of life including our 35,000 troops stationed there as well as likely triggering a response by the Chinese or Russians.
At this point I don't think there is anyway to stop the North from acquiring a nuke capable ICBM. Seems to me the only effective strategy will be for the USA, Russia and the Chinese to make it clear to the North that any attack will result in regime change....