Besides we tried it with scammer tags, know what happened? People still got scammed, then blamed us for it anyway, because hey, that newbie with a month old account doesn't have a scammer tag, he must be totally legit or the mods would have done something right?
Like said previously, that's precisely what the trust system is there to do. Trade enough and other people see this and trust you, scam and you get a nice big red warning on your posts in the trading sections.
Most of the scammers are newbies. All the scammers joining are new, so we can't instantly mark them for scamming, can we? There's a limit to what we can do, you realize.