How do I keep trolls out of my forum?

[agilent]

I am having trouble with trolls joining. I can remove them after they join, but they just rejoin using a different screen name and email address. I can IP block, but many of them use connections which have changing IP addresses.

Is there any identity software or some type of verification software or hacks, etc.. that might discourage trolls from joining?

I am sure some of you have had problems with trolls so I would like to get feedback on the best way to keep them from joining to start with. I rather catch a troll at the front door than to have to remove them after they have already been posting and creating a disturbance in the forum.