At the very bottom of every page, you will notice this:
user contributions licensed under cc by-sa 3.0 with attribution required
This pretty much explicitly says that you can "steal" posts—as long as you both link to the post and state that your copy also uses the same license. I'm sure there are many sites out there that copy SE's posts and properly satisfy both of those conditions, but, unfortunately, there are also many sites out there that copy SE's posts and pretend that they're their own. (If you do happen to see a site like this, please refer to this post on the subject.)
So, they do not in any way try to prevent people from copying content. The only thing they are concerned about is unlawful duplication of content. In terms of trying to prevent that, I'm not sure what they do—other than ask people to report it when they see it.