Any other ways I can deal with a friend who doesn't stop being annoying.

I've officially had it. I've got a best friend who likes to send individual letters as messages on purpose to p**s me off. For instance, if he sent a message saying Oh no you don't it would be "O H N O Y O U D O N T ! ! ! ! "

I've spoken to him loads of times to tell him to stop as it p****s me of and he say he will stop but goes straight back to it.

This morning I woke up at 5:00 am in a happy mood excited to start the day until I turned my phone on to find 100+ WhatsApp notifications from him only doing individual letters this morning. I got into a fit of rage and threw my phone across the room (luckily it's not broken).

I've turned of all notifications on my phone, the last time he done was 10 minutes ago with 18 messages I've told him to p**s of and leave me alone but his continuing to.

Is there any other things I can try? He won't stop and as you can tell I am really in a foul mood because of him.