You can't. You can only hope that you can learn to accept these as a veiled attempt at cheating, control your jealousy, and hope she changes. Or better yet, move on. She's giving you all the signals that she cares more for her past, than having you in her present. It's all so obvious. It's time for you to choose. Sorry for being so blunt, but I really don't like hearing about people being used by others.
You say woman so assume you are not one of her parents. Therefore she decides who she contacts. If you don't like what she is doing you should talk to her about it. If she is open and you can see who she her conversations with these people then you have nothing to be concerned about. If these are private conversations then she is telling you in a very indirect way that you are not exclusive. Again that is a conversation you should have with her, not complete strangers on the Internet.
If your GF is talking or texting ex's or other guys it is b/c she's missing something in your relationship. Women who are happy in their relationship and are treated the way they want to be treated don't cheat. Men cheat for the thrill and excitement of something new (grass is greener), woman however cheat searching for something they had and want back (emotional needs mostly). You shouldn't be looking on how to stop her b/c you can't, you should be asking yourself why she feels the need to do so??!!