I wouldn't go as far as prosecuting, but in severe obesity (like Tom was) then I think it is a very mild form of cruelty, there are a lot of health risks, and there are also the effects like not being able to wash themselves properly, and not being able to play - plus, it can shorten their lifespan. So I think JAck's suggestion of going to classes is the best way to do it - but not sure who would do them, I know the rescue's vet have overweight cats, not sure about my vet, I only know she has a three legged cat.