You called?
I’m definitely all for finding ways to make the software do the work but I agree that this sounds like a behaviour change issue.
How about trying something simple first like posting one of your threads and making the messaging something like “We have a whole lot of new members on board these days and I’d love to hear their voices. Here’s a question for them. etc”
Then if the others answer you could respond with a light hearted "You are definitely not one of the new voices 
" . A few of those might start to get the message across.
It could simply be a case of them not realising what they’re doing.