Thats really gross D
There must be some way of communicating to these guys as to exactly how they must wash themselves in the pre-booking shower. Guys are results driven. They are there for one thing only and if they know what needs to be done before they can get into the bedroom, then I suggest the vast majority will comply. Maybe a standardized text or email pre-booking or a sign in the bathroom. You may have to be blunt but so what , provided its not made personal to them !