You are the only one who can stop them - Do not answer unknown calls and do not call them back. Consider getting a call blocking device (or app for your cell phone)-they're available online and start around $50 or so. http://800notes.com/nb/search?q=call+blockers
Also take a look at: http://www.wikihow.com/Block-a-Call
They can't be stopped. There is no way to prevent calls fro being initiated. However you can invest in a call blocker . Also do not
pick up on call numbers you do not recognize.