Start putting up signs with the special dye packs (that they use in bank robberies) embedded behind them. Just randomly place them. As soon as one of the "kids" tries to take one... Poof!!!! The threat of being teased because you are now Barney Purple will be enough of a deterring factor. Especially if the other kids post it on YouTube and it goes viral. Oh, and have the rule that no purple people (especially Barney when he sings that irritating song) are allowed on Metro property.
Beyond that, how do you stop "kids' from being "kids"?