Is it horrible that I don't want my boyfriend to be friends with his ex
It sounds pretty normal actually. Why don't you want him to be friends with his ex?
People told me that she was my enemy or whatever.
You should be careful about putting a lot of trust on other people, they might be trying to cause trouble in your relationship. You should trust your partner and that he will do the right thing unless he does something to break that trust.
I don't want to hear it. She must go.
Maybe you should use this as an opportunity to show how much you trust him. If he hasn't done anything to make you suspicious, it would be good to give him the benefit of the doubt.