I remember a discussion in one of the technical boards not long ago were a seasoned member speculated that the newbie who started the thread might be an alt of someone wearing a signature. Posing a question with an alt, then engaging in the discussion to increase his post count. I consider myself fairly skeptical and suspicious, but that suspicion seemed darn cynical to me. I understand your concern, but the restrictions you're proposing seem like "throwing the baby out with the bath water."
The first proposal is to create a board dedicated only to newbie members, and that board should be signature free, but the question is how to force newbies to post there, unless they were denied access to post anywhere on forum. This is probably a bad idea, but it may have potential with some modifications.
A dedicated newbie board would become cluttered pretty darn quick. We have boards dedicated to specific topics for a reason, a board like the one you're proposing would be all over the place with no structure.
The second suggestion focuses more on a topic opened by a newbie member to be automatically locked in let's say 5 answers in the event that the topic launcher does not ask a new question or reply in any other way. I know it is possible that the answer to a question is not given in 5 answers, but if we look at the Beginners & Help board a specific answer to the question will probably be in the first 5 posts.
This is a discussion board, and restrictions and limits on discussions would be counter-productive in my opinion. I'm not saying that I think you're wrong, but what your proposing would only hurt legitimate newbies' ability to ask questions.
The other thing to keep in mind is there very well might not be a malicious sig-spammer behind it at all. This isn't the only forum I've seen where newbies show up, ask a question, and never return to acknowledge a resolution. There could be many reasons behind such behavior.