well they cant have unlimited IP addresses right? and they cant change timestamps? so just cap a certain amount of hashes per ip per hour.
i dont think its an unsolved problem when you look at what others have done with abuse in account registration or DOS attacks. where there is a will there is a way
Some of the earlier mega miners were using VPS computing power, which would be trivial to get different IP addresses.
There is a limited amount of programming time, solving what might not even be seen as a problem instead of working on features is a balancing act.
I don't begrudge the big miners, if you want to spend the fiat buying mining computers versus buying coins it's your call. But to me, ten thousand users each having 50,000 coins is not a preferable situation.