A colleague asked me recently: "What am I to do when a 'hijacker' arises in the middle of an important group dialogue and seems oblivious to the thread being discussed or the tone and register in play". As a leader, or facilitator, it's helpful first to form a hypothesis as to what might be going on with this individual/the group that's leading you to read them as an individual hijacker rather than as an agent of the group.
Is it actual, imminent or just imagined by you out of your own concern for control?
Can hijacking behaviour be constellated from within the group in an unconscious collusive way? You bet it can.
But it's not always this way. Rule no 1a (somewhere) - don't take group process offline. Consider asking the group to make sense of what's happening at the very point of disruption. True, this then elevates the matter altogether. In something with such potential heat around your authority I would usually not ask the group what it then wants to do (unless you're pretty sure of the answer, or the choice is relatively benign).
If you do raise a process issue involving said hijacker's behaviour and have a discussion, then ask the group "ok, we've been talking about this for a while; we need to move on, what are the possibilities for action?" This leaves you room to go with the group or for YOU to say "a third possibility is x, and that's where we will now head". Time to stand and stretch. And then regroup.
Rule 1(b) - the exception to rule 1(a). I had a participant a couple of years ago who arrived at the venue of an internal leadership program as a boorish, belligerent, angry man; permanently in oppositional mode, invariably in first position, no EQ, deeply narcissistic. [Poor man, so much energy being taken up to protect himself from who knows what.] His walk talk partner came to me in despair after a couple of WT sessions. He was selfish, ungenerous and was, frankly, getting in the way. He was also a senior manager in the organisation whose program this was. In this instance, and the only occasion on which I've done this, I took him aside and told him to change his behaviour or he would have to leave the program. My judgment: his behaviour was not being constellated from within the group, he arrived in this state and it was now destructive of our purpose. Interestingly, he raised our conversation in morning review the next day, apologised to the group and made some effort to turn up differently (and constructively).