If you use FB, then WhatsApp (Meta/Facebook) already has all your contacts. Adding WhatsApp would not give Meta any more data about you than you have already allowed them to have. There are settings that let you block people.
If you don't use FB, then here is a detailed description of how not to allow contacts and still use WhatsApp on iPhones and Android. Essentially the person you want to get a hold of would have to contact you first.
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