In fairness, most of my clients turn up clean and fresh. I did insist on a client recently having a shower and it was literally like a 4 year old with paint, the amount of mess. He used all of the fresh towels and literally just threw them everywhere. I've had the problem before, but he literally took the biscuit. I buy a couple of different shower gels, I supply fresh razors and shampoo. But I don't expect a whole bottle squirted all around the bathroom.
I've calmed down over it now, but having to spend a while just cleaning up after clients does wind me up somewhat. I will admit grown adults not being able to flush the toilet after themselves is really irritating and it stinks up the bathroom.
By the way, if someone comes through the door absolutely stinking, I will not see them. Luckily, most of my clients are great like that and won't turn up with worse hygiene than someone who's living rough.
I'd definitely say don't leave all your towels in the bathroom, one hand towel in there, one flannel, and hand them a towel to use when showering.
Throw bleach in the toilet prior to arrival, or get one of those hanging cleaners/deorderizers. Admittedly I've never had someone not flush, but my bedroom is very close to my bathroom, so they know I can hear if they flush and if they wash their hands. I had one gentleman not wash, and told him to go back and wash his hands and dick. Just common decency there.
I will say though, better shower gel spashed around than urine. Cleaned up more than a few misaims over the years, civvie and work. I wish more men sat to pee.