To be honest, if we have to ask a poster several times not to do something we do infract for failing to comply with a moderator's request. I don't feel a concrete rule should be in place as some older threads get bumped and are quite interesting, but we do have to use our discretion if we feel that a thread is bumped for the wrong reasons.
westcorklad (23-11-10)
Somebody made a suggestion some time ago (it was'nt me) that threads should automatically lock if this is technically possible once they have'nt been posted on for a specific period of time. This would avoid the bumping of threads for malicious reasons, but it would also mean that new members/posters would not be able to contribute to a thread which had become relevant or topical again.....apart from starting a new thread and maybe linking to the original one.