Menen - an interesting incident. Did the FO discuss what went on with the Captain after they had got off the aircraft?
My answer would have been perhaps "Well, sir, I was doing my job since having called non operation of the speedbrake twice and you failed to operate same I assumed that you had become incapacitated and therefore operated the speedbrake myself to ensure safe flight/landing etc."
If this did not get the required result (ie at least an apology!) then I might be inclined to inform said Captain that I would be discussing the incident with the company's training staff for their opinion!
But remember to get his cards on the table first - maybe there was a good reason why he didn't operate the speedbrake (doubtful I know - but you need to find out WHY he didn't before stating your case!)