Maybe you've tried this, but I would research and then purchase for her a few different types of "substances" that are similar in consistency to poop while being appropriate to handle, such as play dough, fake snot and the like. I know for a fact that they have kits with which you can create fake snot, for I have worked with one with my ten year old daughter. You two could make the snot together and play with it together - just read the box for age appropriateness.
I would then be very firm in forbidding her from partaking in her present activities and give her consequences that she will understand, such as time out or, taking away a toy, so she understands that it is not acceptable in your household. or mine! :) Having created some alternatives, this shouldn't be quite as difficult. Then I would monitor her "on the low" and give her an opportunity to demonstrate that she will adhere to your expectations.
Finally, I might see if there is a science museum or program (maybe zoo program) that deals with feces on a scientific level with gloves and containers and disinfectant. Exposing her to it in that way may help her to classify her activities differently (with direction from you) and to understand time and place. You could create an environment at home similar to the one at the museum and allow her to explore the items you purchase that way.
These may seem like crazy ideas, but hey, I'm a crazy mom.