The US doesn't have a switch to "turn off" a country's reserves of US dollars, I don't know where you get this notion from.
(Eurodollar are one way to bypass it tho)
There's no US precedent, but US government could do something like India's 2016 Demonitization: require new notes, exchange current notes for a limited time only, also limited per person. This might not completely wipe out foreign physical USD holdings, but it would probably make a dent; logistics alone for replacing all the bills would make a dent.
That said, I don't think there's any reason for the US government to do such a thing, and it would most likely lead to significantly lower confidence.
Of course, the underlying value of what you can get in exchange for them can change, but that's true for every medium of exchange in the first place. There's nothing magical about gold, silver, or any other commodity, nor is there anything special about tying a piece of paper to a commodity. If the government doesn't want you to use it or any derivative products as a currency, like what happened in the US with the 1933 Coinage Act, you're pretty much out of luck.
Why, again, should I care about whether or not my dollar can be converted into silver if the government can, overnight, revoke redemption and outright ban ownership of the underlying commodity?
If you keep printing out money faster than the economy grows, the money will depreciate. If the money depreciates too much, people lose a lot more money on treasury bonds. When people stop buying US treasury bonds, the government can't spend more/pay their bills.