De-energizing the circuit is always the first thing to do.
Duh....if it is possible to do so in a timely manner. I am guessing that the window is less than 20/30 seconds.....right? That's not a lot of time to consider options. I think, in most real life situations, the best thing to do would be to free the victim. The knock him off with a 2x4 thing doest make sense. Pry him off, maybe. Knock him out so he doesn't suffer...maybe.
If the circuit disconect is close and accessible of course you would turn it off. If the offending wire is visable and small enough to use your dikes, do it. If there was a way to safely short the circuit and trip the breaker, that would work also.
If you don't have the means to deenergize you may need to go to plan B, remove the victim from the circuit. This may mean kicking the ladder out from under him if he is stuck in a grid ceiling. It might mean grabbing him by the clothing and pulling him free. Just because he is locked on doesn't mean you will be. He is seriously grounded somewhere but if you remain ungrounded you will not get hurt. (I am talking about low voltage here, not that crazy power line stuff that shoots thru your shoes and sets you on fire).