Threadjack
To take over the content of a message thread by changing the subject of discourse to a topic outside the purview of the original subject and/orforum, while maintaining the subject line. A form of amusement for trolls. Threadjacking is distinguished from flaming, as flames are a quasi-personal attack on a poster or on a poster's style of discourse, where threadjacking is deliberatly steering the discussion offtopic.
Arm4war keeps starting threads about how fighting style affects gameplay, but artboy keeps threadjacking his posts with rants about non-real world physics.
Threadjacks
Third person singular of threadjack
No one threadjacks our forum on the possibilities of inter-galactic peace through (extremely impressive) multi-part harmony singing!
threadjacker
A person who intentionally changes the topic in an email or bulletin board thread.
"Jeff was trying to tell everyone about his new computer, but Mike turned it the thread into a conversation about operating systems. Jeff got threadjacked bad; Mike is a serial threadjacker."