B y Julie Zhu and Xie Yu HONG KONG, Dec 17 (Reuters) – Chinese officials are planning to ban online brokerages such as Futu Holdings Ltd FUTU.O and UP Fintech Holding Ltd TIGR.O from offering offshore trading services to mainland clients, the latest development in a broad regulatory crackdown that has roiled a wide range of sectors over the past year. The Nasdaq-listed Chinese firms are two of the biggest players in the sector and a ban would block millions of retail investors in mainland China from trading securities easily in markets such as the United States and Hong Kong….