But making BTC hard to cash out is a hard problem. Although particular addresses can be blacklisted, mixing services are now mainstream. Some return fresh BTC from miners. Even so, it's problematic to mix humongous quantities. For example, the Sheep Marketplace owner/thief overwhelmed Bitcoin Fog with 10^5 BTC. The trail went dead after that, but he got busted while cashing out. His girlfriend was receiving huge international wire transfers, and could not explain where the money was coming from.