You're on siege? Goodmin.
I like, every few maps, to send out a public message saying something like "Press Q M U/D to upvote or downvote maps!" It makes players feel like they're more in control of the rounds, and it reminds them that you're there. Also, I find pith is best when it comes to admonishing - especially if you want to avoid any back and forth from some players. My favourite line is "Don't teamkill."