Any thread bombarded with posts just to to increase someones total post count. The "Joke of the Day" thread comes to mind, but others have been ruined in similar ways.
Based on the fact that there simply shouldn't be nearly as many as there are in horror, my answer is easily and supremely: inbred, or incest-performing, super-strong, backwater mutant redneck hicks. This worked exactly once, Just Before Dawn. Once was enough. (Twice if you count the original Hills Have Eyes. Were they inbred? I don't remember.) EDIT: I responded to this thinking it was "most over-used threat." Like: what subgenre has too many movies? Not changing my reply now. (Apart from adding this addendum.)