[ethbro]

I was thinking about this the other day. From an information perspective, it shouldn't be impossible to design the system you describe (including the implied nuances) because spam usage patterns do and must look different than normal usage patterns under any system that penalizes new accounts.

Hypothetically, the worst you could do would be astroturf. (aka the US/Chinese military style "slightly biased posts from a large number of centrally controlled but seeming unrelated accounts")

However, the idea of slight bias over longer periods is somewhat antithetical to the idea of a spam. In that it might influence you to buy Sparkle towels (honestly, with Amazon prices that low and shipping that easy... [meta :p]) over a competitor, but isn't going to convince you to navigate to {insert sketch get-rich-quick spam scheme here}.

Weeding out astroturf is an entirely more interesting problem though...