Filtering messages that are sent to the list is the easy part:
list.name must be replaced with your list name, which can be hard to find. But click on a message to the list, and from the drop-down menu (next to the Reply button), select Filter messages like these. This will give you a pre-filled Create filter dialog with the list name already set.
But you want to only filter replies to the list, not the first message in each thread. To do so, you would think that combining
list:"list.name" with a subject match on
Re: would do:
list:"list.name" subject:"Re: "
... but this will also match messages that happen to include
re anywhere in the subject.
So the best solution is perhaps modifying the settings for the group. If you're the group administrator (or can influence the group administrator), you could set the default Reply to option to something other than the default, which is Users decide where their replies are sent:
By changing this to To the author of the message only, the sender must make an active choice to spam the whole group, by CCing the group address.