>>pg+(OP)
The nuclear option: Periodically take the site down for a while and then rebuild the community (kind of like the Matrix). The quality people will likely stick around.
>>noblet+Y2
Perhaps only users with a certain level of karma have it roll over to the next version of HN. Everyone else gets reset (or even loses their account).
>>noblet+j4
I think I'd be more tempted with the opposite. Either way a week's break for HN a year would certainly have an interesting effect on the community.