Unfortunately I don't think there is a way for you to do anything about it. If someone replies to that group message it will automatically go to everyone on the list. The person replying can opt to only send it to you, but if they just reply that's what happens.
I had a BAD thing happen with that last month. I was at L&D for my blood pressure and my family was texting me like crazy to see how I was. My husband was home with our kids, so I included him, my mom, MIL, etc in the updates I was sending out. My Mom was driving my h crazy texting and calling him every 5 min. The last message I sent out was to say they were sending me home, and my h replied to what he thought was just me (he didn't notice the group thing) "Your fuc*ing mom is here now!" she just showed up at our house and h was annoyed....well, guess who got that message too? YUP. everyone including my MOM. That sucked. So be careful with those group messages!