Well, this is simple. Make it so (alive) chars of the same account can't be kinned (aka, no access to claim unless they use criminal acts) and homesteads of chars from the same account can't be built if another one is in a certain range (and i mean a quite large one, like several minimaps).
Cause we can't avoid having alts, but at least we can make their life more difficult (since they're used as a workaround for permadeath).
(sure, one could still use multiple accounts, but it isn't like we can do something about that)