You create a New Members board. You only allow them to post on this board for 10 posts or so. Once they have proven to be competent members, they can take advantage of the rest of the community.
You're welcome.
I don't think this will solve the problem. They will just post useless things until they have 10 posts, and then post whatever nonsense they want to post on the other forums. You need to somehow force (not so much force but give the incentive to) people to contribute to the community before they can start new topics.