This becomes really easy if you look at the comments as if they were going to start fading away automatically by default (let's say after a week or so). Would that be disruptive to the content of the original post, or the users who search out that information later?
Comments are supposed to be there to request clarification or suggest improvements to that post. Period. If the comments are not actively working towards integrating those improvements directly into the post, it's time for it to go.
Comments are not supposed to be used like a miniature discussion system or a personal chat room. When they start being used that way, we simply say "sorry, that is outside the purpose of this site."