Depending on your area, Verizon offers a few useful call blocking features...
(if you get an error when trying to use any of the features below, call Verizon. You may need to order it or it simply may not be available in your area)
For Traditional/Copper Landlines: Call Block (*60) (not available in all areas, and only block a few numbers)
Anonymous Call Rejection (*77 ON/*87 OFF)
Call Intercept (call block list and intercept anonynous callers)
(Call Interept is grandfathered, if you currently have it, don't cancel it you will not be able to reorder)
For FiOS Digital Voice: Call Block (no star code, login to MyVerizon to manage list) Anonymous Call Rejection (*77 ON/*87 OFF)
NoMoRobo.com (login to MyVerizon to setup)
If Verizon's available features are not sufficient or unavailable in your area, there is equipment you can purchase to block calls yourself. Panasonic and AT&T for example make cordless phones with a built-in call blocker. You can also purchase a standalone call blocker box. I have personally used the Digitone Call Blocker Pro Series and have found it to be very effective. I do HIGHLY suggest if you purchase any type of equipment to block calls that you fully read the users manual. Understanding how the call blocker works will help you use it to its full potential and avoid blocking wanted calls by mistake.
Lastly, Although it's basically useless against the illegal spam callers, if you have not done so already, you should get on the National Do Not Call Registry (DoNotCall.gov). It will get rid of the unwanted callers that follow the law.