Do you weight it on the number of posts, or the value of the contribution?
Simply using the number of posts might promote spamming for the purpose of gaining influence, but then the value of the contribution requires a preliminary voting system that first allows users to determine what weighting each post should bestow upon it's creator -- and then you need to decide whether you weight those votes....
Ownership belongs to the collective posters according to the number of posts in the thread so far and thus deletion of a thread should be based on a complicated voting system based on weights.