I don't want my boyfriend to talk to his ex
What is the reason why you want to prevent your boyfriend from talking to her.
I do not like her and she is kind of crazy.
While I understand not wanting your boyfriend talking to an ex, you should be careful that you don't tell him what to do. Instead, tell him how you feel about him talking to her and see if he understands your concerns.
I don't feel like talking about it. The ex is going to talk to him anyway.
Just be careful with coming across as controlling to your partner. This may cause your partner to shy away from wanting to be with you since they feel their freedom is restricted. If it is something that bothers you, simply tell him how you feel about it without ordering him to do anything.