If there is an automatic "reply to individuals" response - with "reply all" an option - that might help. Some mailing list programs have that.
It encourages people to think about if they want responses to go to the whole list - and if they should.
Some people might have to go back and reply to whole list because of an error; but it's better than seeing a lot of posts just as well sent to just one person and continuing among two or three people.
Thanks
Cm in DC