1* scam other because others simply believe them - they are here so long with positive trust and history
2* ROI - yes, this sounds really stupid but for sure somebody is buying accs just for longterm profir from sig. campaign (without any bitcoin interest at all, just for "profit")
there are not so many ways how to deal with it, except more strict moderation and focus to some "special" examples. don't want to mention them here, but they are guys with more then 14k posts within two years or 6k within year..
Forum stats >> Top 10 Posters:
https://bitcointalksearch.org/user/hilariousandco-164822 : 15293 posts @ mod wearing a sig ad (so all this post are paid) #register date: November 20, 2013
https://bitcointalksearch.org/user/amph-99297 : 13007 posts # register date: April 13, 2013
What are we talking about ?