For the Banco Delta Asia type of sanctions to work the US must be willing to play hardball with Chinese banks and cut them off from the international financial system:
The United States does not rule out imposing sanctions on North Korea similar to the 2005 financial restrictions that had almost cut off the communist nation from the international financial system, the White House spokesman said Monday.
“I wouldn’t rule out additional steps like that,” White House press secretary Josh Earnest said at a briefing in response to a question whether unilateral sanctions the U.S. is currently considering against Pyongyang would be different from the 2005 measures.
At that time, the Treasury Department blacklisted a Macau bank. The measure not only froze North Korean money in the bank named Banco Delta Asia, but also scared away other financial institutions from dealing with Pyongyang for fear they would also be blacklisted.
That resulted in the North getting almost cut off from the international financial system. [Yonhap]
You can read more at the link, but One Free Korea has a great analysis of the sanctions options that Congress in considering.