You'll be able to block a user you do not want to play or chat with by going to their profile, tapping on the three dots at the top right corner of the screen and clicking on "Block".
To unblock a user, you'll need to follow these same steps and tap on "Unblock".
Bear in mind that when you block a user, they won't be able to send you match requests or messages in the game.
In order to protect your privacy, the user won't be notified when they are blocked, and there is no way of knowing if you have been blocked as well.