Chinese-language online black markets have become one of the biggest drivers of cybercrime in history, processing tens of billions of dollars of illicit finance every year. Helping the trade in stolen data, money-laundering services, and even at times electrified shackles, these underground marketplaces have fueled scam operations and human trafficking around the world. Now, officials in the United Kingdom have dealt one $20 billion cryptocurrency marketplace a potentially punishing blow.

On Thursday, the UK’s Foreign, Commonwealth and Development Office, also known as the Foreign Office, revealed financial sanctions against the Xinbi Guarantee online marketplace, which will likely limit its operations. The online bazaar, which has…