This is actually not such a big problem. Bitcoin's accounting methodology uses something called UTXOs (unspent transaction outputs). It gets complex fast, but s

This is actually not such a big problem. Bitcoin's accounting methodology uses something called UTXOs (unspent transaction outputs). It gets complex fast, but short version is that if you send me some bad bitcoins, I can separate the good bitcoins from the bad. The problem you mention comes up in the other direction - I am a money launderer and I have some bad BTCs. I can use the UTXO system to mix it with good BTCs I control, creating questionable BTCS. This is is why there was recently news of bitcoin mixers being fined: https://cointelegraph.com/news/us-financial-watchdog-fines-early-bitcoin-mixer-60m-for-money-laundering https://cointelegraph.com/news/us-financial-watchdog-fines-e...