I just ran into someone with BO yesterday. Walks in, reeks (wall of BO hits my nostils enough for me to pause and realize the idiot reeks), leave BO in the room afterward and a scent trail down the hall.
Report it to whoever controls the practice rooms.
Stick a sign on the door or entry way. If you stick something on that door, put the same sign on other doors. This way you're not specifically pointing to one room/one person (and people might think it's you who stinks or that you're the one who put up the signs). But you'll probably Mr. Stinky to see the sign, and he may be using another room besides your practice room.
Plastic on the bench? If it's that bad (and mention to the room controller that you used a towel and BO stuck to that) maybe try a different bench.
Lysol (or bleach? but that's harsher I think) on things before you use the room. Maybe a fan or some way of airing the room out.
One thing I tried once that back fired on me was to put cologne under my nose. Problem solved, right? You don't smell the BO anymore. And that part is true. Mission accomplished. Except I realized it created another problem when I still started smelling BO when I did that. BO so bad it made it through the nose cologne. And then it dawned on me that I was sitting in a spot with BO that foul. And yes, it did stick to my clothes.
I'd be blunt with a little threat in the signs. They stink. It's a problem. They do have 15 min to take a shower. If they don't clean themselves up, you'll start being more specific which room, time, and person description it is. And mention that it's been reported to whoever controls the room.
Maybe request whoever owns the building/rooms have the custodians come through and clean things up. Tell them it still smells days afterward.
You might be able to loot and use paper towels or something from the restrooms. Let those collect BO while you practice and then toss them out. If someone sees you and asks, they're probably not going to say much when you say someone has BO, it sticks to clothing, and you have to practice. If it's the building owner, they'll be more aware it's an issue.
If you want to be more discreet, maybe leave a note on the piano Date/time, and that the room stinks/please clean up your act.