Trolls Don’t Go To 4Chan
The best way to attract trolls is to declare war on trolls.
Can you imagine a more tempting target?
Declaring yourself a troll-free zone may assure existing members you're taking action. But it's going to attract a lot of trolls.
Deterring trolls isn't a secret. Reduce the impact and increase the cost of trolling.
The impact includes the reaction from their post. If the post is quickly removed, no-one reacts, and few people know it occurs, the trolling was pointless. Declaring war on trolls is the opposite of this.
Increasing the cost is an efficiency process. You can approve all member accounts. Only allow newcomers to post in specific places (or make a limited number of posts). You can have members or moderators help one another to remove trolling remarks. Twitter's new approach wasn't bad. You can pursue legal action too.
Reduce the impact and increase the cost. Pick both.