Spam FaceTime group calls

I have been getting spam FaceTime calls where there are 20-30 people on the call, all phone numbers and 1 icloud email. They will call repeatedly 10-20 times in a row, relentlessly. I have gone through and blocked every number but somehow they are still able to call, which seems odd. I saw another question with this same issue but the response received was just to block the numbers - which the person also indicated they had done already. This doesn't help. I have not been willing to answer, concerned for obvious reasons what may be waiting on the other end. What can i do to make this stop? Is there any way to figure out who is abusing Apple tech to spam and/or prank others? please help.