You don’t downvote, you flag a post.
Press the flag symbol under a post or if you’re on mobile press the … And then the flag.
If enough flags are received Discourse will close a topic.
What you’re seeing now in terms of posts being closed and opened is down to the way I brought the old posts across to Discourse.
I’ve just sat and cleared 3200 community flags (downvotes) and they are now working their way through the system.
It won’t be like that when we’re operating normally. Only the odd thread will be closed now and again.