Centralized control over the ability to perform transactions is one of the problems that Bitcoin seeks to solve. If you transact completely within the Bitcoin "system" (i.e., I send you bitcoin as payment for something, you send me the something, you keep your bitcoin then use it to buy something else), no one can stop you. The Bitcoin ledger—the continuously updated record book of "who" (more precisely, what address) owns how much bitcoin—is distributed amongst thousands of computers scattered across the world. Neither the US government nor any other entity has the power to alter the ledger or stop anyone from transacting on it. Granted, it's not yet practical to conduct all your commerce within the Bitcoin network, but that is the end goal (i.e., no need to convert from bitcoin to dollars). The points of conversion between bitcoin and government-controlled fiat money (e.g., exchanges such as Coinbase, Kraken) are the weak points where the government could in theory freeze/seize one's money and does in reality enforce tax reporting. But those vulnerabilities are really a problem with the fiat money system (not with Bitcoin itself), and those vulnerabilities also apply to your checking/savings account, brokerage account, 401k, etc. The government monitors much of your normal financial activity (crypto-related or not) as a matter of course, can subpoena any additional information it wants at the drop of hat, and has been known to freeze assets on very dubious grounds.