I think they both have their benefits. White-listed users would presumably be very unlikely to decide to turn to random spam - that's why they're whitelisted. And blacklisting can never completely stop abusers because abusers can just open another account, and another, and another, getting increasingly bitter and thus more spammy. It can stop accounts, not abusers.

A balance of both is the best option, but the more manually dependent these systems are, the harder it is to scale anything up... Without hiring a factory of staff for $1 a day.