It makes more sense to look at the military for preventing anything, they have assets, training, knowledge and skills to actually do something. The federal government is in at least 20 trillion of debt, they cannot help you if the world stops buying their promissory notes.
Its really the other way round, citizens contribute alot of money to government and Washington still doesnt balance the books. If things go bad the government want even more of your help not vice versa.
Outright nullifying and invalidating the dollar would mean killing the US economy and likely pave a way to another Civil war.
Really the point is if the tide changes we dont get to choose which way things go. Dollars are favoured right now so alot of presumptions rely on that status quo. I would put your scenario another way, what if they banned Dollar cheques. No business can write cheques any more, would it be disruptive sure but not impossible to trade.
What if we banned all credit cards from usage in dollar transactions, even more upsetting to business but work arounds are possible. How about we banned every physical cash note from use in trade, no dollar notes can be passed across a counter only alternatives. Terrible, some people would have to alter everything but we have alot of digital bank accounts and so its possible for people to learn a different way.
What if we switched off all dollars transactions of every type, bonds, notes, digital transactions, credit cards, cheques and government issued promises including cash. Terrible, disrupting, loss of liquidity, asset destroying, bankruptcy inducing but not impossible to trade any goods at all as theres a hundred different currencies out there. Every credit card can operate in foreign currency, can settle its accounts with you, every shop that takes credit cards can trade with anyone who has a card even when dollar is gone. I imagine 90% of customers would lose alot of their wealth but it'd still work on the 10% left.
Dollar is king, dollar is the centre of the world but change is possible and its not always a choice for any of us.