You can't.

If the threat of expulsion from the relevant communities is not sufficient (and if the local population accepts the Greenbacks as payment for services rendered), the only way to stop them is sending in the gunboats.

But of course, if they are forcibly ejected from the €-zone, they would have no particular reason to issue Greenbacks instead of just printing their own money.

- Jake

Friends come and go. Enemies accumulate.