Graffiti is never a nice thing to see in a bathroom. I saw this in a hotel and it worked quite well.
- Paint your stalls black
- Put a small chalkboard in the stall with chalk.
- Since it is way easier to write on the board, people will write on it.
- If things are written that are offensive, they will be erased by those who are offended.
- It can be easily cleaned every time the restroom is cleaned