Whatsapp — irritating messages from unknown source

I am repeatedly receiving irritating messages from unkown person. He/she is using different numbers every time i block the previous one.. I have managed to note down this no. +[protected] all previous numbers lost while unblocking to get info. Please trace out this person and share his/her identity. Please set an option for users in whatsapp to recieve messages only from saved contacts in our phone. Or else make your system more robust to avoid fake identity persons who are using whatsapp as a platform to annoy people. This is my generous request so that i can still use whatsapp as a secure medium and think not to discontinue using this app.