Its a nice idea, but when I've been reporting spam accounts, a lot of them have been setup sometimes 6 months before hand. You'd think they would forget and move on, but some clearly are pretty organized.

There's something called account nurturing. It's done here, on reddit, even in small local forums. It's not a %100 solution but it would slow down the spam. At least people would be more inclined to contribute before spamming. But, again, vote rings solve that as well. Need x amount of karma to post? Easy. Invite only would make it harder to join but won't address spam accounts that are already inside.