You should ask them! Maybe they'll start thinking about it.

I've had a few occasions where I've gone after someone who left droplets after themselves. I (discreetly) brought them into the bathroom and kindly asked them to please clean up so I can use the toilet. Perfect amount of shame having to clean it up while knowing I'm waiting, in dire need for a wee.

Never had to repeat myself either, but I don't invite or visit people who would have to hear that more than once. (Except for children and perhaps teenagers, they can need a few reminders if they insist on standing).