Hacker accounts will be blacklisted by exchanges, his XEM will be useless.
I do not believe any human will or would be able to contact all places trading XEM before said coins are moving? (if true what I'm reading here - that there are stolen funds)
edit
Wow
Lon Wong has appeared to confirm Coincheck was hacked, calling the stolen funds "the biggest theft in the history of the world."
CMC
https://coinmarketcap.com/currencies/nem/#markets shows 32 markets, and the number of exchanges is less than 32, so it's not that many.
The hack is in the news everywhere, and the exchanges could easily track and blacklist the addresses. Question is if they're really going to do this