Breakups are always very painful and sometimes, when you end a relationship, you risk your ex partner turning against you and trying to ruin your life. Don't worry, I've been through it too and here's how I got out.
Steps

Step 1. Annihilate it with your kindness
When he sees that his actions don't bother you, he will end up leaving you alone.

Step 2. Ignore it
If he tells you something bad, in person or via text, don't say anything or pretend you haven't heard it.

Step 3. If they ask someone to tell you something, act as if you've never heard them

Step 4. Completely erase it from your life
When he's around, act like he's not there. If you really want to make him angry, when someone mentions him in his presence ask "who?"

Step 5. Never be bothered
If he should call you ugly or fat, don't believe him! He is just suffering from your separation. I'm sure he never believed it, not even for a moment.

Step 6. Don't try to take revenge and spread some gossip about him
You will only make things worse.

Step 7. Act like nothing has happened between you
It will drive him crazy.
Warnings
- Give a definitive cut to emails, messages, calls, etc. It might sound too cruel, but it will allow him to move on faster.
- If he's threatening you or if things are getting out of hand, talk to a trusted adult, parent, or teacher.