If you disagree with a post, click Disagree.
The only glitch I see with that are unintended consequences. For instance if Jane Doe posts "I'm an idiot", and you know she isn't, there will be a temptation to his 'disagree' that will show up as a negative I presume when it in fact was intended to be affirming. I may be assuming wrongly about how that would show up on the post though.