Am I allowed to prevent my husband from hiring a female worker in his private office to avoid unlawful seclusion (khalwa), or should I just explain the matter to him and leave the choice to him?

If a woman's working with her husband leads to unlawful seclusion (khalwa), then it is not permissible for him to employ her. You have done well by advising him. Continue to advise him kindly, and if you offer advice, you have fulfilled your duty. It is proper to continue advising as long as there is hope that it will be beneficial, based on the Almighty’s saying: "Therefore remind, if the reminder profits."