I do a lot of riding through local parkland, crossing a few driveways to get there. I do my best to make sure that my horse doesn’t leave piles of poop where someone will have to drive through it - either on the paved park roads or on driveways. (Horse poop on trails is a different matter.) When my mare tells me she’s trying to poop, we hustle to somewhere grassy ASAP. If she leaves a pile, I will dismount and kick/scrape it off the road. The effort is a basic courtesy, I think. If I ever left a pile of manure in my barn owner’s driveway, I’d be in trouble, so I don’t see why a neighbor’s driveway should be any different.
Safety can be an issue, re-mounting can be an issue, sure. But in where I am, losing access to property and trails is a huge issue too, and riders leaving manure in places it shouldn’t be (and insisting that anyone who has a problem with it is being unreasonable) contributes to that.
OP, I would have a friendly chat with the stable in question and just let them know you’d appreciate it if they could stop by and clean it up after their ride if it happens again. If you have some kind of fixture that would help shorter riders get back on afterward (mounting block, a ditch, even a big flat rock) I think that would also increase your odds of having a rider dismount to clean up.