The ol' rock salt blast!
Now for a little unsolicited advice: In Maine the use of force, deadly or otherwise, has to be the equivalent of what force is being used against you. If you're in a fist fight, you can't pull out a knife. If somebody is using deadly force against you, you can use deadly force to protect yourself.
But if somebody is damaging your personal property, breaking into your car in your yard, messing with a cache, you will end up in jail if you blast them with rock salt, guaranteed. If there's a great concern that somebody would mess with your cache, I wouldn't place it. I've done most if not all your other fun caches, have there been problems other then the coin swiping we've been discussing?
Sometimes the road less traveled is less traveled for a reason.