Oh how awful - I can't remember which site in York you're on, but it's always worth reporting any incidents to the police on 101. Local councillors always look at the local police stats, and if a pattern arises then they can put pressure on the police to step up patrols etc.
I'm assuming that you're on an open site rather than locked. You can't do anything on your plot that might carry the risk of injury to anyone who comes onto the plot, however much you might want to or feel that the culprits deserve it.
Perhaps have words with neighbours, do a flyer round local houses if you are overlooked. And, if you're on one of the ex-council sites, let YACIO know via your site secretary.