I have noticed that vandalism is worse in some scenarios than others. This may be due partially to how good I am doing at others aspects of the game, but I have noticed sometimes I've had a great park with an excellent rating and it's like I got 2,000 gang members walkin around in my park. unfortunately there is no surefire prevention, excpet for what Jeff mentioned about giving all the guards a certain area to walk in. That is better than letting them stroll wherever they want, however some would argue that it takes more time than what it's worth.
quick tip: if you do give your handymen and/or guards certain footpaths, keep them out of que lines as much as possible, I have never seen anyone throw up in a que line, and I usually keep anything breakable out of them.
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