"Zombie" tasks, for me, are tasks that I will probably never complete after creating them. Either I'll never see them again, because I forgot to tag them and I always review tasks starting from the tag cloud; or I'll never complete them because they don't have a due date and I only focus on tasks that are coming due soon.

I should never create "Zombie" tasks at all...but you know how things go...sometimes you get sloppy.

To kill "zombie" tasks, I created a smart list called Zombie with tasks matching isTagged:false OR due:never. My goal is for this list to be EMPTY. Once in a while, I look at this list and try to get it to be empty by assigning tasks in this list either a tag or a due date if either is missing. (Tasks need both a tag and a due date to vanish from this list.)

If I can't figure out what the tag or due date for a task should be then I delete it because I don't really need to do it!