How to make it so people can't iMessage you?

So I changed my number so my ex couldn't talk to me, thing is like me he has an iPhone and can still message me via iMessage, is there a way that I can make it so people can't iMessage me? Like change the email so only people I message know it or something?

In your imessage settings there's an option for receiving, your number is there. You can use your number or email as an identifier. As long as he doesn't know your number he can't message you.