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She just told me to flag if I disagree with the post dude.

That's why I thought the flag option was there, else it would have been called unvote or negative vote or downvote. In my personal use I use it to flag stuff that I feel doesn't belong here, like plagiarism, spam, identity theft, etc. While others may want to use it to disagree on stuff, they are free to do so. I just think its enough not to upvote something I don't agree with or am uninterested in, then go and give my votes to the ones I feel deserve it more.