How do I stop a spammer from calling me?

I have never heard of blocked numbers not being blocked; there is something else going on. Either the caller keeps changing the caller ID or has no caller ID. You should file a police report if you know who it is.

Do Not Disturb let’s you list calls that will connect even whe DND is on, so it is still a viable option.

Enable Do Not Disturb at night.

I’m just surprised that Apple has not figured out a way to block unwelcome calls.

There's nothing they can do about it. No more than any other phone service or manufacturer can. Spam calls (mostly those from ever-changing spoofed numbers) has gotten so bad the federal government has tasked a department to work with phone companies to stop it.

Couldn't tell you why it keeps getting through, but you've been given other options to silence the phone.

It's nearly impossible as spammers have taken to spoofing a different phone number every time they call. Your only solution is to turn the phone off at night, or set it to mute.