BEIJING—China put further pressure on exiled businessman and government critic Guo Wengui, with a court slapping an $8.7 billion fine on his company and punishing several associates who were involved in the takeover of a securities firm.

The Friday court ruling adds details to some of the government’s earliest accusations against Mr. Guo, particularly his dealings with Ma Jian, a long-serving vice minister in the secret police. Mr. Ma’s detention in 2015 signaled a turn in Mr. Guo’s fortunes. Mr. Guo has remained overseas...