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It’s the green speech bubble icon with a white phone receiver inside. You’ll usually find it on the home screen. This method will also block incoming messages from the selected contact. There’s no way to disable calls without disabling messages. It’s the two overlapping speech bubbles near the bottom-right corner of the screen. If there’s no conversation with this person in the list, tap the New Chat icon—it’s the square with a pencil at the top-right corner of the screen—and then select the contact from the list. It’s at the top of the conversation. This displays their profile. It’s one of the red links near the bottom of the profile. A menu will expand. This blocks all incoming calls and messages from this contact.
Open WhatsApp on your iPhone or iPad. Tap Chats. Tap the person you want to block. Tap the person’s name. Scroll down and tap Block Contact. Tap Block.