Can I post a question about some of my code that I want to have a different licence under?
Say I want someone to review my code but have it posted so that a license covers it for not to be re-used or reproduced etc?
As far as I'm aware, if you post code in a question, you're licensing it with CC-BY-SA 3.0. That is, link to it, say who made it, and use for free, commercial applications allowed.
What you can do is post the part you want to get reviewed as CC-BY-SA, then link to some other place for the rest of the code - but all that does is stop people from legally copying the parts not in the question. They can still illegally copy it and use it for their private purposes.
Additionally, all questions must contain enough code for a code review - just a link is not allowed. And linked code serves as context, not review material.
Rebuilding the part of the code as a hypothetical scenario is also not allowed - because it means we lose context and can't properly answer the question.
So no. You can't post code that people cannot use in their own projects.