Christ, reading your posts is chilling.
Please, please, please, tell someone in real life about this. It's for your own safety. If I was one of your friends, I'd want to help you.
Change the locks, google YouTube videos to find out how to do it. You can buy the barrels in B&Q for £10-£20 and just need a screwdriver.
Send him one last message on What'sApp telling him you don't want him to contact you, then block him.
Don't engage with him any more. If you do, he can wear you down until you give in to him. You said you've tried before, but you didn't go no contact. You MUST this time.
Tell your boss at work what's happening, as chances are, he will turn up there.
Ring Women's Aid.
We can only give you support and advice on here, but only you can actually do these things. Unfortunately, we can't wave a magic wand and make him go away and leave you alone. You've done really well so far, but you need to follow it up by following the other advice you've had on here.
All the best. 