You could trade your Bitcoin for Ethereum with A and then trade Ethereum with Bitcoin with B and you get back Bitcoin that can't be digitally tied back to your original source.
It may not be traceable directly through the blockchain(s), but the exchange destination will be, and they're almost certainly going to be complying with local laws which means they'll respond to a subpoena for data and give law enforcement whatever info they have on you, and most exchanges require KYC verification to handle any significant amount of funds.