I'm thinking of getting this game, but I heard that in region play, other players can troll you by purposely building pollution and crime heavy cities to have some of the pollution and crime leak over to your city, and purposely doing a nuclear meltdown to cause part of your city to become radiated.
Well like you said they can make crappy cities but right now they don't really need to do anything because the game griefs you all by itself with the pollution bug that will be fixed in the next patch (or so they say). From what I've seen people generally just try to do their thing and build a nice city and if you're really worried just set your region to private.
I have played a region where someone took a city that had the highway running directly through it that connected my city to the rest of the region. He played only 1 time and his only action was to destroy the highway. Traffic still passed over what used to be the highway but much slower, and with an arcology and the broken traffic agents the city was unplayable.