Most Helpful Girl
Well, you can tell her straight to the point that you're completely done with her, not being rude, of course. You could just fill her in on what's happening in your life, not to show that you're still interested, but to show how good you are doing in your life while she's not in it. Girls can somehow get the hidden message (if they're not dense).
Maybe you can tell her straight to the point that you don't want to talk to her, if you're feeling bold.
Or you can take some time to figure out what you think about this, and reply when you want to. You don't have to be in a rush. :)
But for me, you don't have to respond to the ex text. You really don't. It's not like you're ignoring her, sometimes it's best just to leave things be. And that will send the clear message to her to back off.
Hope this helps. :)