Almost every website will have a "flag" or "spam" button at the bottom or top of the ad. In the case of Trulia and Postlets, there is a flag to "report" problems with an ad. Once you do that, it takes only 24 hours for Trulia to remove the advertisement AND the poster's account. I know because I reported a fake rental offer yesterday and it was down less than 24 hours later.
So use the flag or report buttons, and almost every service provider that I work with here in the real estate world will remove the ad. Trulia just seemed to do it the fastest!