If you block a user, their messages sent in a private chat will no longer be visible to you and you won't receive notifications from them. However, the user will still appear in the team lists and their messages in a group chat will still be visible. To block someone, go to their profile by tapping on their avatar, then tap on the middle icon. You will see an option to Block. If you decide to unblock the user, you can do so by going back to their profile, tapping on the middle icon, and choosing Un-Block.