A cat head on a pike should warn other cats. As would a gypsy head on a pike, a Ukrainian head, a Vietnamese head, and so on.

HAHAHAHAAHAHAAHHAHAAHHAAHAHAAHAH!
I'm really pissed cuz I didn't think of that one!

replace the goldfish with robot fish.

http://news.nationalgeographic.com/n...obot_fish.html

I mean theoretically, you could also then upgrade the fish with some kind of weaponry so they could really defend the pond, but I think the fact they are not edible would eventually make the cats go away. You'd probably only lose one or two, and they would be repairable. Sensor lights around the pond might also help annoy the cats at night and give the fish another tactical advantage, alerting them to approaching kitties.