Downvoting, by itself, isn't going to stop anyone from posting meetup threads. That said, there has been discussion/complaints about meetup spam before, so you're not alone.
edit: clarify wording
I understand. Nevertheless, discussion so far hasn't gotten anywhere. Perhaps downvoting meetup threads would put some pressure on people involved in meetups to resolve the matter.
As of now, I haven't downvoted any meetup-related thread.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.