i was wondering ... what if we circulate coin infinitely to harass central system? like keep exchanging coin between A and B.
Can we halt entire system down?
Any system is vulnerable to exploits. One advantage to having the government run the system is they have the power of law, law enforcement, and enormous resources to stop attackers. The financial system isn't dependent on someone's technical defenses.
There are probably scores of abuse vectors waiting to be exposed. Getting caught exposing or exercising them might land you in a place where you can never have money again or worse.
Well, if this monstrosity ever come about, just run an experiment: try to do it and see how long they let you do it before deplatforming you from the entire economy.