This happened last year with LinkedIn and apparently there was no way one could stop it.
Okay, I'm relatively new to this list: does this problem still occur with any networking sites - Facebook, Twitter etc? There is a way that this can be stopped, as emails are sent from particular domains, we can actually blacklist these email addresses so that their messages can be held for approval, or automatically discarded. I'm CCing in the owners of this list too, as this could be something for them to learn about.