Can I block people on my iPhone?

Before you start telling me to call my service provider, the person I want to block, isn't exactly a number.
If you're familiar with the whole iMessage thing, where if you have an iPhone, you can text other people, who have iPod's, using their email addresses. My ex-friend will NOT stop texting me and it's getting annoying, I've asked her to stop multiple times, but she won't, so I'd really like to know how to block her.

Added (1). I have a real cell phone number, so I'm not texting with an email, and she is texting me with an email.

I know how you feel. You actually can't block it. Ither you can ignore it or you can go to the settings and change the email people can reach you at. Just don't tell her your new email.