You can stop receiving WhatsApp messages and calls from certain contacts by blocking them. Not sure what happens when you block someone? Learn more here.
When you receive an initial message from an unknown contact, you will also have the option to Report Spam. This will report the user and add the user to your Blocked list.
Alternatively, you can simply send a message to the contact you blocked and you will be prompted to unblock the contact.
Learn how to block or unblock a contact on: iPhone | Windows Phone | Nokia S40 | BlackBerry | Nokia S60 | BlackBerry 10
Cheers,
WhatsApp Support Team