I have mentioned this on SoylentNews several times, but I believe the only sensible moderation system is one where you cannot only add or subtract one, but where you can give your exact rating of a comment. Then let the average decide.
And if you are too concerned with single moderations instantly getting a comment to +5, then make it so the score can never be higher than the number of moderations.
And if you are too concerned with single moderations instantly getting a comment to +5, then make it so the score can never be higher than the number of moderations.