Most get automatically detected and I simply don't answer. For the rest of them, once I figure out what the call is about I just talk over them telling I'm not interested and hang up and then add the number to the spam list so that others don't need to answer either. I've never acted hostile towards them and people that do are worse than the telemarketers themselves.
I don't get any after I made one cry. I simply asked them if they were happy with their life, if they are proud of what they do, what if I was their parents or grandparents? They hung up after a watery "shut up". Any I get are dead lines or immediate hang ups on their end now.
Check with your carrier. Some carriers offer free spam blocking services. For example, T-Mobile has an unpaid tier of their “scam shield” service that you can turn on and configure through their app.
I don't say anything when answering an unknown number. Real callers will always have some background noise, while spammers will be dead quiet and only assign a scammer to talk to you once you trigger the mic.
Useful for when unknown callers are expected - applying for jobs, etc.
I haven't received any after I threatened lobotomy with a chainsaw. What they're doing isn't legal; are they gonna run to the cops with those threats?
The above is actual spam/scam calls, such as the ones from "microsoft" about a problem with my computer. Yeah, I'm on linux.
I'm a bit more lenient about the people who call me about something annoying and similar - They're just doing a (legal) job. I save them the time and hassle by interrupting their pitch with a firm "Not interested". Sometimes I ask how they got my number, as it's illegal to cold call s9meone here under certain circumstances. I've managed to get off of a few lists this way.