They still have the knowledge, and the country has its own reserves of uranium, plutonium and rare earth minerals. You've certainly delayed their WMD program, but you haven't ended it.
Invade? Iraq's been a 14-year slog, and that's only 375,000 soldiers from an ethnically-divided open desert. North Korea is 7 million soldiers, in rugged mountains, who've spent every day for the last 70 years being told the US will invade and their sacred duty is to kill every last invader through brutal guerrilla warfare. Imagine Vietnam if the VC has spent three generations training for it.
Assassination? Kim Jong Un technically isn't even the leader. North Korea is a necrocracy. He's simply a caretaker for dead leader Kim Il Sung, and he's not even that prominent. They've already had two dictators suddenly die, with no challenge to power. He'll just be replaced by someone who will continue Kim Il Sung's teachings.
Don't play into his hands with a direct military action, which is the narrative the entire regime is based on. Force him into an asymmetrical battle his regime didn't spend 70 years mentally and militarily preparing for. Discreetly cripple his infrastructure in a way that undermines popular confidence in his leadership and Juche beliefs.