Tap FRIEND MENU button to enter Friend menu
Tap FRIEND LIST button to see list of registered your friends
Tap [x] button next to the friend who you want to block
Confirmation message will pop up. Tap [BLOCK] to proceed
Final confirmation message will pops up. Tap [BLOCK] for final confirmation
Now chosen player should be blocked and out of friend list. There is no indication to the other players so your block won't be recognized by others.
Your name will be left on blocked player's list, but you will never receive any invitation from them.