I remember when I was in high school there was an outbreak of bathroom vandalism. It was a pretty nice private school and in general we did not have problems with vandalism. But one year graffiti started showing up in the bathrooms.
Almost as soon as it started an all school assembly was called and the principal spoke to all the students. He got furious! At one point he even called us gutless. It was pretty intense at the time but the speech worked. The graffiti stopped as soon as it started.
So I don't want to say that screaming at people is the best strategy, but maybe taking a hard line and setting clear consequences. People can be scared straight.