In a blog or other community forum, the action of writing or posting comments which serve to create a fuss or controversy, but do not help resolve the issue being discussed. When you "deadletter" a public discussion space, you kill the opportunity for effective discourse on relevant subjects and kill the possibility of progress on those issues by diverting attention and energy to tangential or non-sequiteur issues, esp. using incendiary, passive-aggressive, or thinly-veiled remarks aimed at personal or public/political targets.
"That guy totatally deadlettered the issue at hand." (or) "Please ignore that deadletter post about the upcoming event; the important issues we need to cover before the event are really these..."