Putting an individual contact's calls on do not disturb
There is a 3rd party app called YouMail. The free versiin of it will allow you to set different conditions for different contacts. You can change which voicemail message someone gets. You can send a certain contact directly to voicemail. And you can also prevent callers from being able to leave a voicemessage. I had someone who would also call me 4-8 times in a row and no matter how many times I asked him to stop and explained why, he just kept doing it. With him, I would vary between the silent ringtone strategy mentioned by Tyson and YouMail’s ditch call.