They are a threat, and it has nothing to do with touching the American homeland. The threat is that the longer tensions remain between NK and SK, and the longer NK remains unstable, there's a possibility that war pops off and the US will be dragged in (to defend SK). There are more than 30k US troops in SK right now who could quickly be killed if any hostilities start. We don't even have great war game success against NK. Sure, the US would "win" of course but a LOT of people will die.
There's not much we can do. Can't unilaterally attack, can't really negotiate...NK has the nukes already lol.