@Alexandra1141 There's no way you can hold anyone responsible, since 3 guests are using the area. Coffee stains on the wall don't sound like a big deal to me, they sound like normal wear and tear. You have to expect that things won't be kept in pristine condition when you do short term rentals.
I'd suggest you add an easily cleaned surface to the wall near the coffee station. Or repaint the wall with a high gloss, oil-based paint which can also be easily cleaned.
I'd also suggest you add a photo of that kitchenette area to your photo gallery. That's of more interest to prospective guests than all the outdoor shots. If I were looking at your listing, I'd assume that there was no place to prepare any food or coffee, even if it's mentioned in the written info. Be aware that many guests don't bother to read through all your listing information, they just look at the price and the photos.