The cat just needs to stay in a separate area from the hamster.There is no other choice,unless you'd like to have a constantly stressed hamster that eventually will have a severely compromised immune system
Cats(well,the wild relatives if you must be specific,but they're the same species regardless)are natural predators of hamsters.I think a lot of people often forget that cats are obligate carnivores,who do not have the same human emotions regarding their prey.They can and will torture animals to death with no remorse,and that is their nature as horrible as it sounds.
As for keeping a cage up high,that's not a solution either when even overweight cats can jump as high as refrigerators and such.It really must be a completely separate room which cats are never allowed into,nothing else