If a users attitude changed towards a user, then they can unlike the "Content" because they don't like that user because of some disagreement or what ever. As your saying your liking the content and not the user, so why should you unlike a post you previously liked if the "Content" has not changed.
Liking a post is Simply yes i liked it for its face value.
Liking a user a attitude / emotion can change.
Liking and unliking, is more about an individual than a posts content.
Maybe a meet in the middle would be enable unlike button ?? On Off toggle.
IMO modern forums are so full of negative comments between members it really hard to keep it positive, underhand comments that we as admin dont always see or know about until it all kicks off.
Likes could be linked to upgraded subscriptions rather than post count. If the user puts the time and effort into a post then we can show our appreciation as a "Like" and it should stand.
Also enable like in this forum would be a good option to have also, as i have a jokes section, and if there getting likes just for copy and paste stuff its not much effort for a like and then does not really show there a contributor with respect from others.
Food for thought.