I just promote peeps who talk about their own experience when they first join. Bots and marketers don't talk about themselves except as a foil to their products. Statistically, some of them will stick around, right?
If we trick them into becoming productive members of the community first, isn't that just a win-win?
That would be the best spam solution ever; you convince them that if they are thoughtful for just a little bit longer, then they will be free to spam.