Well, I've had it. I must get at least a dozen "please join my crappy network" invite messages each week from Ning networks. I haven't yet found a way to prevent them from "inviting" me, and all the emails violate CAN-SPAM in a number of different ways (no "unsubscribe feature", no physical address).

Their FAQ says

“To protect your privacy, only your friends and certain members of social networks you belong to can send you messages on Ning. If you’re a member of a network, the Network Creator and Administrators can send you messages and you can message them back. Further, if you belong to any groups, the Group Creator and Admins can get in touch with you. In order to send another member of a network a message, you’ll need to ask them to be your friend first.”

But apparently any Ning network can send me emails like "Come join me on Lame Group on SpamNetwork...".

Just this morning I got 6 different invites from "Members Only Network", all within the space of 10 minutes.

So for now I would suggest not joining anything on Ning until they address their growing spam issues.

Edit: Ok I found the culprits -- the spams were coming from some of my Ning.com "friends." *sigh* I knew there was a reason why too many friends was a bad thing.