I would ask my client-PRIOR to the workout.. to “please focus on your workout as it’s crucial to stay focused. AND as you trainer it’s my job is to insure a safe, correct, workout with your goal/safety first and foremost in mind. If I feel you need to stop talking and re focus I’ll let you know during our session.
Are you ok with this?”
I have asked my clients to stop talking during crucial exercises/times. It can be said in a nice manner, a caring manner. To this day they are all still training with me! It shows you are a pro and really care about their results