Just selling them the oil no one else wants to sell them.
Seriously, it’s a good bet that the Chinese oil tankers are private companies, not PLAN vessels so this is the free market finding a way around sanctions, not a government per se.
The solution is easy. Allow the transfer then board the NK vessels for inspection. If they have oil say you’re going to enforce the sanctions and sink their boat.
I remember watching the newscasters breathlessly reporting that giant Chinese armies might fight each other at Tiananmen Square.
The Chinese Army killed some protesters.
Funny how that Chinese Army seems to come together to do the communists bidding.
Agreed on all but your last sentence: Tow the tankers to S. Korea or Japan; sell ships and oil to finance further surveillance. No need for a big oil slick... ;-)