I'm a student, living with four other non-students, who've all recently graduated. There's one girl, let's call her Sarah, who officially doesn't live here, but effectively does; she stays around all the time, smokes cigarettes and weed in my housemate's room (that's another issue entirely) and eats food in our house, with our plates and mugs. She has a habit of leaving cigarette ends (butts, and sometimes just ash) on those plates and keeping them in our other housemate's room. I'm disgusted, of course, but all of my attempts to ask her to stop doing this have failed.
So at first I posted on the Facebook group politely, twice, asking that this not happen. My housemate (Claire) apologises on Sarah's behalf, the first two times, and promised to talk to her about it. It's Claire's room that Sarah smokes in (Claire also smokes and does weed, but at the least knows how to use an ashtray,) so when I brought it up to Claire in person after the third time it happened, she told me not to worry, because she yells at Sarah every time she does it. I've asked Sarah in person, politely, not to do it, and she's just sort of given a non-committal "Oh yeah, sorry, I won't do that anymore" response.
I've come down this morning to find more ash in cereal bowls, and I don't know what to do about this anymore. I'm really angry at her, but I'm also effectively powerless to do anything more than what I've already done. The problem is, not only is Sarah Claire's best friend, she's also the girlfriend of two of the male housemates (it's a polyamorous house.) While neither of her boyfriends smoke, and I know that they both also dislike Sarah leaving ash in plates, I'm not aware of either of them taking active steps to discourage Sarah from doing it. Also, all of them besides one have been living here previously, so I can assume that Sarah's probably been doing this since before I moved in. So I risk pissing off at least three of my housemates if I get directly angry at Sarah.
I'm really sick and tired of Sarah's behaviour - repeatedly using dishes for her ashtray. It's disgusting and unhygienic, and I feel powerless to do anything about it.