"Threadjackal" is a term that has a history in some "thrilling threads of yesteryear." "Threadjacking" can also be a useful term.
threadjacking (threadjackal) - when a thread's subject is completely changed from the original post's intent. As in: starting with an article/post on birth control and ending up with a thorough discussion of zebras. Those who engage (intentional or not) in such behavior may be accused of being a "threadjackal".