I've definitely engaged people (fellow pilots and controllers) on the network in PM conversations that have led naturally to my mentioning VATSTAR, but I've never simply cold-called someone via PM to urge them to visit and sign up and I would absolutely not condone anyone from our organization engaging in that kind of petty business. I wonder if it's something the BoG would be open to modifying the CoC to expressly forbid stuff like that?
It is a common practice for me to PM a link to new pilots with Pilot Training information (VATSTAR.com) if I see that they're struggling. I have an alias for it. It's something to the tune of "Aloha. If you'd like to learn more about flying on VATSIM and want to improve your flying skills, I highly recommend that you check out VATSTAR.com. You can obtain pilot ratings, etc..." and so on.
Messages like that from a controller to a pilot, in my opinion, are contributing towards the improvement of the network by better educating pilots and creating a more professional environment, which in turn makes it more fun for everyone.
VATSIM's three fundamental principles are education, professionalism, and entertainment.
Rob, I don't think that your PMing with pilots and controllers about VATSTAR as you describe is the same kind of spamming that we're dealing with from other organizations. I believe that your desire to improve the quality of the members of the network is different than someone trying to hire 500 pilots for a VA. Same thing with what I do: I see a pilot that needs assistance, so I help him/her by providing the information necessary to become a better pilot.