I don't know what SiteLock Prevent is, I can only help you with Nabble-related issues. All you have to do at this point is keep a CNAME record pointing to "n5.nabble.com" (without the quotes) in your DNS configuration.
Can I remove custom domain from my forum myself, or do I always ask you?
This is not easy. Ideally you should fix your DNS issues and the sitelock problem you mentioned.