Assuming you're talking about wood being stained by poop, here's a cheap and easy solution- hydrogen peroxide.
Readily available in mass quantities, cheaply, it should do what you want.
Poo is essentially full of undigested proteins, which are staining the wood, presumably leaching into the fibers of the wood.
Scrub the affected areas with peroxide and a scrubbie brush. It will bleach out most species of wood, especially wood high in tannins, like oak.
Then, scrub the remaining areas of wall lightly to blend in the dolor disparity.
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