How can I keep this sign from being stolen?

This is the sign in front of our Zen temple. I suspect that the thieves are students from the nearby university who think it’s “awesome” to have a sign that says “Zen Center” hanging in their frat house. This is not the first time, but I’d like to make it the last.

I thought I had made it too hard for a drunk undergrad to want to mess with removing it. The sign is ½” thick Corian. I had used the strongest industrial polyurethane adhesive I could find to glue two brackets (made from massive aluminum angle) to the back of the sign. Then I used 8 huge lag screws to attach the brackets to the 4X4 wood posts. The posts are embedded in 300 lbs of concrete.

The weak point, apparently, was the adhesive. They must have taken a crowbar to it.

It will cost $600 to replace the sign, and unless I can come up with a better deterrent, I fully expect it to disappear too.

Can you creative people help me outsmart these punks?