Most of us probably don't know how strangers literally get our phone numbers and contact info to offer promotional products like vacation packages, insurance or credit card services. Chances are, they got it from some random thing we filled up at the grocery.
So to avoid them, we have to start by making sure the forms we fill out are stamped with something that says, "The information you share is safe with us and won't be given out to any other entity."
And then, when some do come calling, be polite and firm in saying that you're not interested and will appreciate your information not being kept for future reference, what more forwarded for other promos. I repeat, be polite but firm. After all, they're only doing their job and their job is to call people and try selling them things and services online. If anybody gets rude or obnoxious, however, make sure to get his/her name and where he/she reports so you can properly complain about them.
And yes, blogging or e-mailing people about the incident does sometimes get their attention. But only sometimes.
And don't share friends' names and contact info if you're not sure they will appreciate it. The least you can do is ask a friend first if he/che can be bothered by such calls.