When a thread goes off topic rather than just hiding the posts, open an appropriate thread or move the posts to a more appropriate thread if one exists. This is common practice on most fora. I would be surprised if the tools do not exist already to do this.
Also I can't help thinking that the word hijacked has negative connotations. Whilst personally I am not bothered by it, those that have a need of "a safe place" to express their ideas might wonder about such emotive language and I can't help but think imply another's intentionality.
Finally ... fora are places or a 'fiery furnace' to strengthen and refine ideas, concepts and beliefs. So long as this is done "respectfully" I essentially see no problem.
Edited by romansh, 18 April 2016 - 08:13 PM.