Yes, you can prevent them from switching companies after having received the training. Specifically, you may have these trainees sign a training agreement with your company, indicating that the trainees will pay all the costs related to the training if they switch companies within 2 to 3 years after they have completed the training. It would be better to have the training agreement, with all necessary signatures, notarized at a notary public office in China. Training agreements with reasonable terms have been upheld by Chinese courts.