How do you go about blocking such calls?

I use an app by the name "Should I Answer" Seems to work pretty good.

There are a variety of ways ranging in price from reasonable to free. 3rd party boxes, phone systems (landline and cellphone) with blocking built in, computer apps

Do a web search and keep an eye out for "First Ring Suppression." Your phone must ring the first time to ID the number which is then blocked or allowed. FRS silences that first ring on all calls. If a blocked number calls, the phone lights up, reads "caller blocked," goes dark and never a ring is heard.

Silence is golden

IOS has a feature that you can enable "Block This Caller"

You must have the feature from the phone company.If you have it,dial *60,listen to the instructions will tell you everything about the feature. Or call and add to your phone service.