What would be the best way to get rid of the North Korean ruling party and de-militarize the country?

Negotiate a Peace deal between the North and South and get them to agree to a merger, Promise the Kim Family protection and pardon everyone.

Lift Sanctions if they agree, and help with the merger of the armies and beurocracy, For that to happen you will have to provide a temporary period of self control to the north so that both govts can work in parallel till they merge.

Conduct fresh independent elections and bar all existing rulers from participatin, the new United Korean Govt. would look over the coalition and control the army till both haves finish all the legal work

United Korea exists, and Kim family is provided land anf laws are passed to censor any defaming speech against them.

After a few decades the law is removed and the existing Kim descendants are guilted into donating their private land to the govt. due to public pressure after the crimes of the Kim family are made public knowledge.

Till that time United Korea will have censor and control South Korea's Internet access like the North does.

This would be the Promise along with land and a full pardon that would get Kim to agree to signing the merger.

We'd stab him in the back by revealing the truth once he's dad or too old to do anything.

A massive anti communist and Anti DPRK educational campaign would be held and new laws would be passed similar to Germany's anti Nazi laws after WW2

Once the Koreans abhor their DPRK past the laws would be relaxed over time before finally being scrapped in secret so that freedom of speech can reign again.

The end.

Also we'd demilitarize the North to make sure China doesn't oppose the merger.

since

United Korea would be Pro-USA,

Assasination

Let it collapse on its own. It is not our job to control them.