Navigate to the Craigslist post in question. Hover your mouse in the top-right corner of the post and click the reason you want to flag it -- examples include "Miscategorized" and "Prohibited."
Click your browser's "Refresh" button to re-load the posting. Click a different reason for flagging the post. If you clicked "Spam/Overpost" the first time, for example, click "Miscategorized" this time.
Return to the ad periodically to see if it has been removed. Don't inundate Craigslist with flags for the same reasons -- if more than a day has passed and that ad is still up, it means that Craigslist hasn't found sufficient reason to remove it.