An admin-approval move will be coming in version 1.5 (to be released in a couple months).
Besides that, there are plenty of spam controls already in Q2A. There is a CAPTCHA for all anonymous posts. You can allow only registered users to post, or registered users with their email confirmed. I use this on my site with a lot of traffic and have not had a single spam post since I started.
You can block users or IPs, and limit posts by users or IP as well (e.g. only 5 questions per IP per hour).