Trolls need two things:
1) Anonymity. It's hard to be a troll if your real-world friends might find out what you're doing. To prevent trolling in gaming communities, I've contacted ISPs, employers, friends/families to suggest their colleague/son/student stop (it's quite effective).
2) Isolation. It's hard to troll people you like. A study from Wi and Lee notes those that gain high standing within a trolling community are less willing to to go extremes. Those more isolated, troll more.
One trolling community, Wi and Lee note, bans use of nicknames and hanging out in the forums. Nicknames and building connections reduces trolling (which is the goal of a trolling community).
Think 4Chan here. You have no way of really knowing who else is a member. You're anonymous and isolated.
We can apply the opposite to help reduce trolling. Reduce anonymity (not to be confused with pseudonymity). Require use of a consistent identity within the community.
Force early connections. When someone joins the community, force early connection with other members. Encourage the off-topic discussions. Try to build early, strong, connections to other members in the community.