It's disgusting to see something like this, you cannot understand the mindset of the people who do this for fun and simply do not give a moments thought to the upset, damage and cost that they cause, so sad.
I know it's an extra expense but have you thought about using something like smartwater as a deterrent and a possibility of finding the little scrotes responsible?
http://www.smartwater.com/home.aspx
I hope it all gets sorted in time