The real spam prevention will always go back to reputation and web of trust. Even people writing for urbit understand this. The idea that $10 solves everything is silly. Ultimately the $10 thing (and even the limit to the number of planets) is impotent in the face of natural reputation.
> By convention, booted addresses are expected to have some existing reputation outside of their name alone, since they’ve been used on the network. Reputation, good and bad, comes in many forms. Did the address operate any useful infrastructure? Did it get placed on any blacklists for spam or abuse? Did it simply send and receive messages? The ability to programatically track reputation is still in its infancy, but we expect the tooling to develop as urbit grows.
> By convention, booted addresses are expected to have some existing reputation outside of their name alone, since they’ve been used on the network. Reputation, good and bad, comes in many forms. Did the address operate any useful infrastructure? Did it get placed on any blacklists for spam or abuse? Did it simply send and receive messages? The ability to programatically track reputation is still in its infancy, but we expect the tooling to develop as urbit grows.
https://urbit.org/blog/value-of-address-space-pt3/
This is to say that you do not need limited address space or even a cost barrier to have spam prevention.