There is no way out of the box to restrict the creating of new topics in Questions for Confluence. We have a request open to include that functionality in a future release:
Nic makes a good case for not restricting topics on this thread:
"But topics and labels really should be free-format. They're quite informative as they allow the user community to freely contribute to the list - they add information for all users and can inform the administrators and business users of what is really going on and how people are really talking about content instead of having a fixed set of jargon forced on them."