I've been cleaned out over the past few years too, was recommended a plastic Eagle Owl, about 12 months ago, rather than the Heron and.... So far so good (i know i'm tempting fate by writing this!!)
Had all the advice to net over the pond but for me its a garden feature, the fish are secondary, so i'll see how the owl gets on and if the flying fecker makes another raid, it's back to the shop for more cheap fish... My dads old pond was netted and it ruined the whole look for me, though he did have some expensive fish.
Oh yer, I have heared that putting a 'pergola' over a pond can deter