Negative reps do nothing. If you want to let someone know you agree with him/her the "thumbs up" is perfect. If you want a poster to know you disagree with him/her, normally you would tell him/her why.
alternatively, if you dislike, disrespect or even hate like a hater, someone, some goddamn pinhead sh!t-for-brains, think they're a mouthfarter, want to key their e-car, make fun of them in front of their girlfriend, stick your tongue out at them, make them...well, mad...if they made a sarcastic remark to you 7 years ago and you hold a grudge, if they nitpick your spelling, if they think pot is cool and you don't, if they're a heathenous sinner and you a God-fearing pius e-pope, then perhaps you can 'get them back...'show them who's boss'...that sorta thing.
It's all pretty mature and stuff.
Oh, and remember, they don't do anything.
They don't do anything.
not a friggin' thing.
but sometimes you'd think they were akin to some d!ckhead killing your dog, best friend, stealing your job or boning your wife.