Let me get this straight. He loves you so much, he can't live without you, and he also badmouths you to your mutual friends?

Look at it this way: the more he badmouths you, the less you will be tempted to let him back in!!

Here is an idea: tell people that you are taking the high road and not getting involved in a word war. This will be healthy for you and also it subtly reminds them that he is taking the low road, and they will also be reminded not to believe everything he says.