You can make your ads somewhat flag proof by flagging them as best of Craigslist. You have to flag each ad as best of craigslist a few times from different IP addresses. You also have to make it appear natural.
I know of two ways to do this. First, you can use a proxy to get different IP addresses and then flag them yourself. Or second, you can pay to have random people flag your ad for you using www.microworkers.com
Before you do either, you might want do do a Google search on how to stop Craigslist ads from being flagged. Last time I looked there were a couple of good youtube videos on the subject.