I'm glad you started this thread, so I'll let you know my opinion.
Passion for dead guys is an extremely specific niche, and there's no room for competition. Your forum is immensely better by all means (board software, number of users, possibilities), and this leaves no hope for other forums to grow up. They can exist, of course, but are going to remain a hopeless minority (Hey, the other forum doesn't even support attachments!)
On the other hand, if you use the terroristic approach of banning, I think these are the results:
- You give importance to the other forum, looking like you're scared by it;
- You appear like a tyrant and a part of the community starts hating you;
- You literally donate users to the other forum; since they can no longer access CDG, they will start a new community elsewhere and help it to grow up.
In other words, by banning users you're helping competition and reducing your power!
So I would allow users to sign up wherever they want, and let natural selection do its work. CDG has nothing to worry about, since it's the big and better one.
If you want to punish someone, then go for those users who steal content from CDG and post it elsewhere - But, everyone should be free to join any forum