Look for opportunities to productively challenge trollish comments or make statements about civility. It respects the participation of the more constructive commenters.
What does your audience want to talk about? How can you tap into their outrage, pride, nostalgia or excitement? Also, if appropriate, try kicking a conversation off by having your staff share first.
Look for opportunities to show that your people and your organization are rooted in the community you serve. Also, writing informally, with some personality, can encourage people to interact.