I really don't know what can be done about such idiotic morons. We had some at my work too, there were two groups there of course, the cat lovers and the haters. Our friend Debbie, she has been talked about here previously in regards to stray dogs she has taken in, was really going all out for the cats, she had a really nice big feeder for them and some of the guys took it and threw it in the dumpster. That is when I got involved. I told some of these jerks if that feeder disappeared again I would find out who did it and there would be hell to pay. It never disappeared again.
Debbie and one of the busdrivers(karen) ended up catching them in humane traps and getting them fixed, Debbie now has 13 cats living in her barn(she lives on an 80 acre farm). Karen took several of those cats also, some were kittens and I believe she tamed them and found homes for the poor things.
Our big maintenance boss was so mad at these cats he threated to poison them. He claimed they were using sand meant for the schools as a litterbox. Debbie told him if he poisoned them she would report it, needless to say she is not real well liked around there, but that is the level of her commitment to animals.
It never ocurred to me that some of these guys could have brought these cats, but now I see it is possible. I wish I had a solution for you, I think you are right, just stay quiet and keep feeding them for now, good luck and thanks for trying......tom