I guess this is more of a general StackExchange question... For many years, MathGroup has been an excellent resource to ask the kind of questions that now can be asked on this site. However, MathGroup has been a moderated group, leading to a ~1 day delay between questions and answers. The reason given for that has always been (and I believe it!) that this procedure is in place in order to avoid spam. So my question is: Why is there no spam on SE?
The main reason is that the community itself takes responsibility to identify spam and flag it for a moderator to clean up.
In the unlikely event that you do see some spam, you can click the "flag" link under every post.
If you aren't sure, you can always ask about it here on the meta site.
Flagging and closing, if found to be helpful by the moderators, contributes to your flag weight. The current value of this quantity is listed in your profile (not sure about new users and beta). The more flag weight you have the more flags of yours draw the attention of the moderators.
A number of badges you may get is connected to this flagging activity, so there are quite a lot of people actively looking for posts to flag (check the Review link in the top of your screen to find suggestions for reviewing). Most spam posts are deleted within minutes this way.