Trent --

Sure you can -- it's the U.S Constitution!

See Ammendment XIV, section 4 --

The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any State shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.

Read the last clause again --

But neither the United States nor any State shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.

All bonds and bills issued by the Confederate government or any Confederate states represented promises to pay by them. Since these governments no longer existed, they could not pay them, and by virtue of the XIV Ammendment, the U.S. government would not pay them.

Any individual or corporation or country holding bills, bonds or otherwise attempting to collect a balance due from the Confederate government or any Confederate state was just out of luck.

For what it's worth, this is exactly why the French, the Russians and many others sought to prevent the collapse of Saddam Hussein's government in Iraq.