There are really 3 ways to prevent spam reliably in a "global" system, ie a commons:

1. algorithmically, e.g. bayesian spam filtering, ... a constant arms race
2. [transitive] reputation, e.g. WoT
3. making it expensive

The more of (1) and (2) you can use, the less of (3) you need. But you probably want all of them at scale.