Stop being so open-minded. This is more like you having holes in your head. At some level she is enjoying the attention from this guy. Tell her that you were a fool to ever tolerate any of this and that you are done putting up with it. Tell her it stops now, or you will not be with her.

Alternatively, you can continue passively accepting this. If you do, I predict she will end up in bed with this guy. That's what he is really after. He may not even really want the whole relationship back (or he would have been pursuing her harder, even after she moved away.) He wants to plant his flag once more, so he can feel she never got over him. That will boost his ego.

Your girlfriend doesn't know what she wants. She needs for you to be real crystal clear about what you want and expect. Stop deluding yourself by saying this is all due to her being such a "caring" person, and stop referring to "this ex-BF thing" as "annoying." This is way beyond annoying. This is you being made a total fool of. If you have hopes of being with this girl longterm, then curb this nonsense immediately. If she is really so ditsy as to have such complete lack of boundaries, then you need to have boundaries for the two of you. Women tend to fall out of love with men who do not command respect.