Dogs can't learn (or un-learn) behavior "after the fact". Once your dog has brought something inside, you can't correct the behavior - you can only correct or teach the dog while the unwanted behavior is happening. So in order to correct this behavior, you'll have to catch the dog in the act.
I would suggest supervising your dog when she's playing outside and coming in. This defeats the purpose of the doggy door - allowing your dog independence in going out and coming in - but once she's figured out what behavior you want from her, she'll apply it all the time, even when unsupervised.
When your dog goes outside, go out to watch her and stay by the doggy door. Let her do her thing outside. When she heads for the doggy door with an object you don't want inside, stop her and make her drop the object. When she does, praise her and/or treat her, and let her go inside.
Once she "gets" what you're asking her to do, she should make the "drop object BEFORE doggy door = good dog" connection and do it independently. You may end up with a pile of objects right outside the doggy door, which could actually be convenient when it's time to clean up the yard!
Edited to add: It occurs to me that this training might get your dog to think that the good behavior is bringing objects from the yard to you, not dropping them outside the doggy door. You'll know if this is happening if you stop supervising your dog and she starts bringing things into the house and finding you to put them down - or if you're not home, putting them somewhere she associates with you (like your shoes, your favorite chair, or your bed).
If this happens, maybe try putting a box or bin of some kind outside the doggy door. She gets the treat when she puts the object in the box. This could prevent her from getting confused about what you want from her. (It could also have the added benefit of teaching her to clean up the yard herself - who knows?)