How can I block unknown contacts from sending messages in Whatsapp? Are there means of blocking unknown contacts from sending messages? Unknown contact here refers the time when someone sends you messages  (contacts displayed are in the form of letters, and are in similar digits like mobile phone). Unfortunately you cannot reply or save the contacts. 

Are there means of blocking this,  only allowing messages from people in your contacts only?
When you receive an initial message from an unknown contact, you will have the option to Report Spam and Block. 

If you just want to block the contact, simply tap on Menu Button &gt; More &gt; Block.

So, in other words, When you receive the message on WhatsApp from a number and that number is not yet in your contact, then WhatsApp will show BLOCK option at the top which should allows you to block that number from sending you FURTHER messages.

To block an existing contact on your Android phone, you can try below steps:

1.Open WhatsApp and go to your Chats screen.

2.Tap 

Menu Button &gt; Settings &gt; Account &gt; Privacy &gt; Blocked Contacts


.
(This page displays all contacts that you have blocked)

3.Tap the Add blocked contacts icon in the top right.

You may Want to check FAQ for more details.