Anyway, I may have found a solution. Basically, you go on Cafepress and order some bumper stickers that protest the act on it. I was thinking for mine I'll put "When did it become socially acceptable to pee with the door open?" I'll use a really dainty font. Maybe the one used in the Ambulance LTD logo. Then you just stick them above the toilet, at what is about eye level for most people. Or on the seat. Just put it where people will see it.
You can also use a sharpie, but I advise against it. If the management wants to take it off (but if it's a guy then he probably won't) then don't make it hard on him. Plus, a sticker looks a lot less "vandialismy" than some scribbles on the wall, so the management will probably be less tempted to remove it in the first place. It's a good idea.
And if you don't want to spend money on this, I've got another solution. Basically you kick the guy really hard every time you go to a stall and find this. Go for the upper back.