Yes, if someone is joining in the topic and contributes something another poster feels is irrelevant they will almost certainly be able to reply and discuss the post or ignore it. Yes we all can, and do, ignore posts that do not interest us. Yes, threads wander and we can all go along with that, either enjoying the wander or bringing it back to the OP or, occasionally, both. These sorts of posts are not 'flaming'. Flaming can even, occasionally, happen by accident when someone misreads or misinterprets a post but a quick apology will sort that out. That is not what I was talking about.
Actual flaming is a deliberate attempt to try to provoke anger - often by commenting on the person or the way they posted rather than the substance of their post - while contributing nothing to the actual topic. Often these 'flames' are posted for the sole purpose of offending or upsetting other users. It can throw the discussion of a legitimate topic well off track and is often designed to do just that.
I am grateful to Laura for clarifying how GNHQ sees such posts.