Change the part of the command that says [YOUR FRIEND'S USERNAME] to whatever your friend's Minecraft Username is, then execute the command yourself. This will give your friend a special block that does exist, but is not meant to be in your inventory or in your hand during the game. As a result, your friend will crash. Put this command in a command block with a redstone clock circuit (a circuit that repeatedly gives redstone power to a command block) to "ban" your friend from your server. What this is truly doing is crashing your friend's Minecraft app every time he enters your server world.