His ex-girlfriend always sends me messages to harass me. I don't know her at all. What should I do?

Don't talk to her. If you are like her, or you show anger, that's exactly what she wants, and if you quarrel with your boyfriend because of this, she will be happier, because this is her purpose. If you are like her, you will lower your status and treat her like garbage.

Even if you give her a message back, her purpose is successful, because she knows that you care about her words.

The solution is to call the mobile customer service: I am harassed by the text message at this number, and I want to catch the text message at this number.