Snam comes in several flavors. First, there are the "will you be my friend" invitations, when someone wants to link to you. New members of social networking services generally unleash a flurry of these to everyone in their address books -- especially on services like ZeroDegrees, where it's nigh on to automatic.
Then, there are the FOAF (friend-of-a-friend) messages that people send. Similarly, there are messages to entire communities on services that host them, like Orkut.
Finally, on services that mediate requests for introductions, such as LinkedIn, there are the "will you introduce me to X" messages. According to Joi Ito, LinkedIn just introduced a feature to let people prohibit such requests from coming to you. For LinkedIn users who want to opt out, you can use this link.