When someone is sending you a message the first time, you will get 2 buttons: Block and Add to contacts. If you click Block button, then you cannot receive any messages from him/her.
If you have already started chatting with him/her, the 2 buttons disappear. And you added this guy on your contacts, you need to remove the contact first. Press the avatar at top right corner, then you get a short profile pops up.
Then press the 3 little points at the right bottom of the profile, you will get a new menu popped up.
Tap the button to block. (If he/she is added on your contact, you need to press "Remove contact" then you can Block him/her)