These are options, but definitely depending on the situation and business scope, relationship etc do nothing without proper legal advice.
If there are no clauses that protect the shareholders of opening another business then one has for my understanding always the right to start a new business.
The best option is to come to an agreement for whatever reason you don't want that person to be a︀ shareholder anymore. A document of share transfer can easily be prepared and records changed for︁ a few usd. Any other option legal consultation would be be recommended, there are enough︂ good law firms in Hong Kong that can give you the right advice and options︃ for probably a few thousand usd.