use call blocker

play.google.com/store/apps/details?id=com.grus.callblocker

https://pictr.com/images/2022/01/07/Bw5rM5.jpg

Blocking --> My Block list --> "+" --> Numbers that begin with --> Type in "+852"

I think you just need to use a blocking app, after about 1 week of using the blocking app, they stopped calling. The blocking app need to be fast one, it reject the call without ringing. If you let the call ring once, they still think there is chance, and the software will keep trying the other day again.