Get outta here sms.ac!

After finally moving over to gmail, I thought Id be free from spam. I still do get spam, but gmail's spam filtering is excellent and pretty much ALL my spam goes into the spam folder. But sometimes, I do check my spam folder in case I have a real message in there. Low and behold, I see a familiar name, but the message came from sms.ac asking me to join on behalf of the sender. Though the sender did NOT know that the email was sent to me. That really chaps my hide. This company is sending out email invites to everyone in the sender's buddies list without the sender knowing it. That's just rude. And why do I have to sign up for their sms text service when I can go to my verizon wireless or email and do exactly the same thing! Sorry Im just very careful with my email address when signing up for things so i dont get spammed. But the question is...how do you prevent others from adding your email addresses to such services that abuse? Thats cool sms.ac, my gmail account knew you were junk. haha.