fish are my thing!!!
dude id treat em like any other animal you dont want in your pond, you can get an alarm system, (there not expensive no worries) you may actually be able to get it where you got your koi, but it makes a really loud sound when ever any thing gets near the pond (meant to scare off birds of prey and racoons, cats and other things) you can also put a net over it to protect the fish,
if it came down to it, you can get a shock colar and lay the wire around the pond so when they get close to it, the collar lets out a warning beep, if they continue to get closer, it emits a small stadic shock
eventually they will learn
because now that they know what great fun the pond is, they arent gonna stop with out training
=)