There is also a trend of newbies who start threads in an effort to advertise a site by asking some kind of "innocent" question about the site. The site invariably turns out out to be a scam, and the newbie is never seen or heard from again.
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 essentially a newbie jail, which theymos has stated clearly he will never re-introduce.
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.
Users who are asking questions so they themselves can answer them will already have scripted the answer they are going to post. Locking the thread after 5 posts won't stop them from posting with their main account.
I don't think there is a way you are going to be able to combat this without catching honest newbies in the crossfire.