No, mere death penalty, or life in prison, is not harsh enough. If one can be sure that the person indeed is a rapist, he or she should be tortured to death in the worst ways imaginable. He or she should be served a nice, tasty-looking meal made of his or her cut off genitals, be informed what it's made of, to add humiliation, and forced to eat it, then the rest of them would later be cut off in little pieces, while they all the time would be kept alive, by various means, by burning the wounds, and through other necessary procedures. The person would have to eat the pieces and skin that is cut off/skinned off of him or her, every day, as well, and when the person is starting to get numbed out from the torture, he or she should be pulled apart while still alive, or mangled - I guess you could be kind enough to let them choose. All this should be done on a remote "torture" island, as an example, and by robots, if possible, so no torturers get messed up from what they do (I'd gladly do it all, though), and so no one in society starts questioning the reasonability of it.
When done with them, you could use their corpses as bio fuel, in special power plants, as well, to be kind to the environment (that is much needed, nowadays, as we know).
Those who lie about a specific, real person having raped them should also be punished, though. Lifetime in forced labour, without any chance for release, may be a better solution for them, though. However, would they ever try to escape, they would be executed.