From Lifehacker, a servicey post about how to deal with telemarketers:
The most efficient way to get the person to stop calling you requires you to say one sentence: “Please put me on your do not call list.” Don’t say “Can you put me on your do not call list?” or “I don’t want to get these calls.” This will lead them to ask why. Be polite, but firm. If they ask why or won’t do it right away, remain calm and repeat, “I want you to put me on your do not call list.”
My go-to is simply not picking up a call from a number I am unfamiliar with, though the writer, Erica Elson, who has worked for a telemarketing company, says that not picking up calls will just cycle you back into the system so that you will get the call again and again until you finally do pick up. Elson also recommends not hanging up on a telemarketer or being rude to the person on the phone because they’ll simply put you back into the system to be called again later.
Also, I try to remind myself that these telemarketers are just people who have this job because it was the only job available to them, and simply saying no and “please put me on your do not call list” seems reasonable enough. You could also block numbers by logging into your account online (I have also done this).