First of all, here's my last reply: Honestly, it's just meant as a joke, but I get how it could be interpreted otherwise and discourage people from commenting, considering different senses of humor, culture and language differences, etc.
No necromancy unless it adds to a conversation has basically been the rule here, though. Actually, it looks like there's no mention of it in the rules of conduct, so I'll add it in. Personally, I would say that threads that haven't been replied to in a few months are "dead", but people are more likely to write comments that add to the conversation in more recent threads (ones from a few months ago) than older ones (from a year/years ago).
As for the template/picture, maybe it would be better to use it for people who have been here a while or repeat offenders. People have actually been blocked for short amounts of time (starting with a week) for being repeat offenders of minor rules in the past, though it's rare.
We could actually change the wording and make the template an official warning for those who continue to reply to old posts after they've been told not to (say they reply to two old posts at least a day after being told not to), so then we could still use the picture provided and actually have it be useful.
for example:
First offense: User receives a friendly reminder not to reply to old posts unless they have something to add. Even if they've replied to multiple old posts, every instance before they receive a reminder is considered to be the first offense.
Second offense: User receives another friendly reminder, but they are told that their next offense will be an official warning. And we could be pretty casual with this, considering sometimes people won't see the message on their wall, or they'll forget, etc.
Third offense: Official warning. Here, we could use the template, which will be re-worded stating that their next offense will result in them being banned for a week, and then that amount doubles if they continue to reply to old threads without adding to the conversation. (it would be added to their wall, not their profile)
I'll make a blog post for this, so we can discuss if further, since we should give "necromancy" a solid definition, and also discuss what "adding to the conversation" means.
edit: We could also use this system for other minor rules, such as edit warring.
But yeah, figuring out how many times a minor offense has to be repeated for it to become a problem can be tricky.