Ban them. I also recommend making it explicit: your last rule should be “moderators have final discretion”.

The hard part is figuring out when to apply rules leniently or strictly. For example, I think users should always be warned, then given shorter bans, before a long or perma ban (unless they break an obvious egregious rule, or they’re a suspected alt of a banned user). At the same time, you do need to kick out persistent rules lawyerers or they’ll drive away good users.

Moderators shape their community. Hacker News is decent (compared to what it could be) because of dang and tomhow. Many communities (especially on Reddit) have become echo chambers, mainly because the mods are too happy to ban people for unspoken and/or vague rules.