They can stop student exchange programs, reduce amount of flights between the 2 countries, discourage leisure travel to Singapore, significantly reduce money exchange between RMB and SGD. There are a lot of things they can do to make Singapore a truly undesirable place for mainlanders to immigrate to.Chinese government can’t prevent emigration.
Keep in mind that during COVID time, a lot of mainlanders in Singapore went home, because they thought Chinese gov't had it more under control. The mainlanders or even the Chinese Malaysians that moved to Singapore are not bound to stay there.