Such a system would fail at HN, if offered by the site itself. Many users have accounts registered for the sole purpose of bypassing “identity over time” recognition, whether by karma and flagging or simply by username recognition. Anyone who perceives themselves to be blocked would just keep registering new accounts to exempt themselves from it, or maintain a collection of accounts to circumvent it. The absence of blocks removes a key benefit of doing so, which helps keep the sockpuppet population down.

(Note that if you see users that are frequently breaking the guidelines, please consider emailing the site mods using the footer contact link, so that they can consider whether further review and/or action is necessary.)