Open registration is fine.
One trick that works pretty well is that new members cannot create threads until they have made a minimum of 100 posts in existing threads.
Except the script trolls were doing it in existing threads and not starting new ones.
I can handle the annoying Emma trolls. They just post words on a screen. Harmless if annoying. The script trolls though...

I'm not enough of a computer geek to know if what they are trying to do is malicious or basically just harmlessly annoying. But they worry me a bit more than the purely annoying Emma-types.