I'm sorry to read this. I know that poop smearing is very common with autistic children. They say the first thing to do is to analyze the motivation for the fecal play. Is it attention seeking or sensory seeking? Is it a control issue? In a world where they have little control, they control their poop.
But since that has a high propensity of making others sick as well as just gross, definitely a habit to curb. If it is sensory related, what other things can be substituted? (like plah doh, slime or kinietic sand) If it is attention seeking, how to work on that? Same with control. This takes some delving into to work on the why and then how to substitute.
And remember, just because a child is autistic or on the spectrum does not mean that they never face consequences. Many kids on the spectrum are very rule bound. You can make very firm rules with consequences.
Then use a picture map of proper bathroom behavior and procedure. Like rules, teach them how and what not to do.
It would help to know and and when she is playing with it. For example, is she constipated so her poop is very hard? Anyway, besides perhaps trying to change her poops (constipation)....try and find a substitution that can be left by the toilet. Something else that will attract her attention.