In the United States, at least, they wouldn't be "legal tender", which means that every entity (person, government, whatever) is required to accept them in payment for a debt. I believe most other countries have similar rules for their official currencies, likely including Switzerland.
Note that the "debt" part is important. You can refuse to take cash for a new transaction, but not one where the other party already owes you money.
I believe there have even been court cases that held that the type of restaurant that requires you to pay before getting your food can refuse to take cash, while the type of restaurant that lets you pay after you've eaten cannot (because that makes it a debt you owe). Don't quote me on that, though.
Note that the "debt" part is important. You can refuse to take cash for a new transaction, but not one where the other party already owes you money.
I believe there have even been court cases that held that the type of restaurant that requires you to pay before getting your food can refuse to take cash, while the type of restaurant that lets you pay after you've eaten cannot (because that makes it a debt you owe). Don't quote me on that, though.