The wallet with the stolen coins is being monitored, and if any transactions are made out of the wallet, they will immediately appear on the blockchain (I'm assuming NEM uses a public blockchain, I'm not familiar with that coin in particular, but AFAIK it's not a privacy coin). Coincheck already notified all the other exchanges not to accept any transactions from the wallet with the stolen coins, so the thieves will not be able to use exchanges to turn the stolen NEM into other coins, or fiat dollars. Furthermore, if the thieves try to transfer the coins to another NEM wallet, that wallet will now be tracked. As coins are transferred into other wallets (either legitimately or illegitimately) a network of wallets will develop that can by analyzed by a social network analysis tool like Palantir.
This is not like stealing cash out of a bank. The thieves will be leaving footprints all over the blockchain, and the more footprints they leave, the easier (relatively speaking) it will be to associate wallet addresses with meatspace human identities. The only hope they have to beat the social network analysis tools is to get their NEM exchanged for privacy coins like Monero, but with the exchanges now on alert, and P2P exchanges virtually nonexistent, the thieves are going to have their work cut out for them.thanks for sharing..