All they have to do is plumb numbers into a computer or robo dialing machine. However, google your name and number, any place such as spokeo that shows up, make them remove it.
Do a search on call blocking devices and phones.If its a cell phone, there are free call blocking apps out there including trac & straight talk.
http://800notes.com/forum/ta-5d4e343675a5629/call-blocking-devices