having talked to the guys on the Wave Team, it is clear that they are still trying to figure these things out. They are just now opening it up to more users and seeing usage scenarios they didn't even think of themselves. I'm guessing spam wasn't really an issue they had to deal with so far, and from what I've seen, there really is no spam on the site yet. Also, given that you invite your own contacts to join a 'wave,' chances are, spam won't be a huge problem as far as I can see.