Preventing reply-alls from Group MMS text messages?
Unfortunately the current version of iOS doesn't natively give us much control over something like that.
As I'm sure you are already aware of, it does allow us to turn off Group Messaging (and MMS Settings > Messages). However, that would defeat group texts in general - not just replies.
.....Maybe the next OS will?
Select details next to the list of names, select the one name you want to reply to. This will bring up their contact information, select send message/text. Then select their cell number. This will start a new thread to just that person.