Well the UN kinda works how it works. With potentially powerful Civs being forced into other 'ways' as it were (look at China atm, although happening over years not one turn!).
Changing from a closed market (like mercantile) to a free market because of other nations influence (perhaps their own civs too).
I agree that you should be able to 'opt-out' of the UN and not recognise it, but then get a negative attitude penalty from the civs that are in the UN (say -2) and if your in the UN you cannot leave, but if you opt-out you can be asked to join again, say every 15 turns, and each time you refuse its -2 until you join, then the -ve goes away once you join.
Also the voting for civics options, you should be able to vote for any civic option when say a vote for Gov is selected you can pick HR, US, Rep or PS and not just a 'yes or no' option to one of them.
What then is your strategy when an opponent computer civ gets voted into office?
Depends what the vote is for. Usually Diplo win wont happen for an AI, as there is a Higher number needed in the voting, and some AI's that voted them into SG wont vote them the win, unless there is a religious monopoly (not usually after Free Religion though).
If i am not an option in the SG vote, i'll pick my friend (or the one whome i like the most, as usually the other civs wont vote for them if they try diplo win and you can abstain also.
Sometimes if your not SG you cant stop civics being chosen, i would suggest some easy wars to get your population higher (and anothers lower) taking 3-4 size 7 cities will probably make you the run away leader for votes anyway, build a few more cities in places you usually wouldnt but could grow to 5-6 and just lump the upkeep (although by UN time upkeep isnt a problem).