Yeah, I can see a huge potential for abuse. Those that don't like a person or their views can all pile on and no-thank an interesting post. That kind of hurts the post and stifles free speech. If someone is being a troll, the only thing to do is skip it, put them on ignore, or report it if you feel they are always off-topic.
I use thanks because you can appreciate a post someone made especially if they have posted something you wanted to say yourself but they said it much better.
If someone posts something I don't like I just respond in the thread to them with what I don't like or don't agree with about their post.
I don't see a reason for no thanks, the rep system covers all that kind of stuff in my opinion.
I opted out of rep so I can just discuss the issue with people and not make it a popularity contest to boost my ego or for others to use it in personal ways to be hateful privately by sending mean notes on the rep
I'd rather get rid of the thanks button then add a no-thanks button.