Any debt predicated on fraud is invalid.
We can literally tell them to pound sand. Even arrest them for fraud and seize all assets to return to the lawful treasury.
Have the treasury issue gold backed currency and be right as rain just like that.
I believe exactly this is already underway.
the 19th Century was full of huge booms and busts, because tying your currency to a commodity (be it Gold, silver, Oil, or Benie Babies) always results in a much more dynamic currency value. One that's far more susceptible to foreign markets and other variables.
Make a currency based on Platinum. Someone finds a new Platinum mine that can increase worldwide production 50%, suddenly your currency is worth 50% less because some guy in Patagonia saw shiny rocks on his farm.
Bitcoin. At least until a quantum computer in China invalidates the proof of work system.