The easiest method of blocking a specific number, or even anonymous phone numbers, is usually offered as a service through the phone company. These services are normally subscription based, but there are other methods available as well.
Every phone company is different, so many have different names for these services as well as different pricing for them. For those who don't want to increase their monthly expenses there are also external devices which can be connected to the phone line in order to block the unwanted calls. Finally, there are also free downloads which are available for your computer which will allow it to screen your phone calls, so long as your phone line is run through it before going to the house phone itself.