Courmisch

I have had a similar problem on a HEL->PEK flight First, a Chinese woman across the aisle wanted the blocked seat next to me. I explained that she should stick to her assigned seat unless instructed by the cabin staff. But then a rude Southern European man just came in from behind and just seat there noting "there is nobody in that seat". I did not dare make a fuss and call the staff. And there went my extra legroom for a red-eye flight

Unfortunately, taking an aisle seat only works if you know that the flight is not full and the window seat will not be assigned (I pretty much need a window seat in general due to motion sickness). Complaining to the staff only works as far as the staff knows about the blocked seat policy and dares to rebuff the "offending" passenger angry. I have not tried, but I can see how that would fail.

I have also had a more minor case of a Chinese couple on CDG->HEL who had separate seats and took the blocked seat because of that. Arguably, if it had not been for me and the blocked seat policy, they would have been assigned contiguous seats though.