I think you misunderstand what Zembla said and what several of us discussed: there is nothing to "do". You do not request for a seat to be blocked, AF does not do that and certainly not reservations or customer service. It is just that the operational team will do it automatically by blocking some seats on the seat map if/when loads permit.

It is always automatic when it happens, never on request, and it is absolutely not guaranteed because it all depends on loads. In other words, if your flight is affected, what you'll see is one of two things:

1) most common case is that you'll see lots of unlikely seats greyed out/shown as taken (for instance, all the middle seats in the cabin, which obviously is not what people would assign), or

2) when you select a seat the one next to it will suddenly show as taken. If you move, both your former seat and the neighbour will show as available again, but the one next to your new seat will show as taken again as though some ghost was following you!

I should add that I much prefer this automatic version than the "we'll block if you ask" version that some airlines do. I generally prefer a system which tries to be fair to everyone rather than one that favours those who are happy to ask (same goes for upgrades, etc).