Wow, what a nosey (and filthy) neighbor you have there. Pummeling through dirty garbage to find goods for her home, she sounds like George Costanza.
Now that I know more about your situation let me give you some pretty eccentric and crazy ideas you can use to hide this secret from Gladys Kravitz over there.
Ok, one idea is to actually cut this diaper into shreds with a scissor. After you do this you should have mangled up remains, put this in a thick heavy duty black bag, and get another bag for what I am about to say. Now I am going to suggest what someone else mentioned, save up all of your scrap food and when it is time to throw this out dump it into a new plastic bag, and place this into the first (With the destroyed Diaper.) Here is another idea to add to this, pretend you have a Cat in the house and buy Kitty Litter, and pour some of that into the bag with the scraps. If you really want to gross her out put something in there that looks like cat doodie, I dunno, maybe chocolate. She may be so disgusted that she will be forced to halt her treasure expeditions.
Also, Like I said before in an earlier post, be sneaky, do this at a time you think she won't be out and about.