Well, what options does one have to defend themselves against intrusive callers?

• The Do Not Call list (Federal or State)
Works, but is effective against ONLY "legitimate" telemarketers.  Political callers, surveys and some other callers are exempt.  Scammers ignore the DNC list.

• Shouting, screaming, swearing at the caller
Nope.  Does not stop calls.  May make you hoarse, and will disturb nearby people.

• Call blocker app, device or service
Could be a good choice (even though the scammers can use fake phone numbers), and some apps are free!
Google "call blocker" for more information, or check your favorite online sales site for a list.
http://gg.gg/blockers
Your phone provider may also offer blocking services.  Check their web page for information.

If you cannot use any of the above defenses, then:
• Do not answer calls from unknown numbers ... let them go to voicemail or answering service
• Do NOT return calls from unknown numbers!  All that does is verify your phone number!  And, in some cases, those return calls may cost $20 or more to complete.
• If you have a "dumb" cell phone, assign the scammer's number to a group named "scammers" (or similar).  Then, assign a silent ringtone to that group.