(Personal opinion BTW)
First What is a "Necroposter"
simply some one who posts on a obviously dead post, a obviously dead post being something either confirmed closed by the poster, far outdated post where the poster has not said anything in a month + or anyone else. EX: a month + old post talking about recent events or old events lets say WW2 and I bring up my favorite colour or anything else that would be considered off-topic.
It dose not count though if you post something relevant. EX: a month + old post talking about recent events or old events lets say WW2 and I bring up my favorite ww2 tank (Ram 2 BTW). (AKA rule #15 Do not "necropost" in discussions that have not had a reply in over a month unless you have good reason to believe that your reply will be of interest to the original participants (i.e., you figured out how to fix a bug) or you have good reason to need to get the attention of the original participants (i.e., you want to report a bug to a mod author). It is almost always preferable to start a new thread instead. (Discussions that have been stickied by moderators are an exception to this rule; it is okay to reply to those discussions at any time as long as your reply is otherwise on-topic for the discussion.))
How to deal with one:
Ignore them or set your list to show only the newest posts. If you reply to a necroposter and complain about it, hen you yourself have became a necroposter, It would also help if mods were to start declaring cretin posts as dead not locking them but just saying it, however that will also bring up unneeded posts, so maybe after someone necroposts. Or just more moderation on the forums in general