As news spread on Wednesday that Hong Kong banks were tightening scrutiny on mainland Chinese clients opening savings and investment accounts, Zhe Ye – an auctioneer based in the southwestern province of Yunnan – was stunned. The former asset appraiser had been planning a trip to Hong Kong to open an account, hoping to buy overseas assets ranging from the Nasdaq and S&P 500 to shares in newly listed companies such as SpaceX. “I really regret not opening a Hong Kong account earlier,” Zhe said,...