False belief...

Users should be encouraged to report scam & spam posts. Posters of self-replicating scams should at least be issued a very clear warning. Such a warning could appear as a bright red banner message toward the top of the website for a week or two. It'll very bluntly state, in whatever language the user speaks, that such scam posts are, well, scams. It should also alert the user that further infractions may lead to punishment. The reason why scam-posters shouldn't be outright banned from the start is that most are probably just gullible children -- it's very difficult to determine which scam posters actually have malicious intent.
Regardless of how it's done, we need a system in place to deter scam posters. I don't think that a voting system alone is quite enough to do the trick.