I'd stay away from vandalism either way if I were you. As for helping, to be honest, you're probably just irritating them. You can spray paint whatever you want on the wall, but chances are, they're just going to pay someone to clean it off, and be on their merry ways. If it's really bad, they might mention what a hassle it was, but no, that isn't combating slavery. If you want to do that, actually talk to the people who can do something about it instead of spray painting their office building or stealing their calculator.