It looks to me that there's actually no absolute or fail-safe method of preventing spam.
This sort of claim is bad marketing if one wants to sell a product that claims to do just that, but I'm sure product limitation discussion will surface if the buyer is sophisticated enough to ask some of the right questions.
As long as the network is useful, there will be those who will use it "improperly." Let's define spam as the improper uses of the network to generate unwanted content. (There are other, more limited definitions of spam.)
Furthermore, if we think of improper uses of the network as failure events, we may be able to apply some work in termporal logic cited in Ken Birman's book (Building Secure and Reliable Network Applications), which basically says that as long as we can always add one more failure recovery method, we will be able to recover from all failures.
In other words, there is no finite solution to the failure problem, in this case spamming as defined above. Of course, this is no excuse to deploy operating systems and computing environments that are more prone to such attacks. Let's face it. Even if the problem has no finite solution, there are environments that are more secure than others. A system's level of security and protection is based on the effort required to break it.
My claim regardging the infinitude of the spam problem is similar to saying that biological viruses are always going to be around.
Viruses are made to take "improper" advantage of the extreme usefulness of certain biochemical structures including self-replicating macromolecules. To fight viral and bacterial attacks, our bodies use the versatile, highly-tailored antibodies to defend our physical perimeter.
I think those who wish to make "proper" use of the Internet should probably use similar methods to fight spam and other improper uses of the network.