Unfortunately, that's a very difficult question to answer; just the nature of the beast, I think.
All I can say with certainty is that it's my job to draw that line. I try to do that as judiciously as is possible. I sometimes make mistakes, seemingly a condition of humanity. It's regrettable, but if one lives long enough one learns to take one's mistakes with a certain amount of humility. If you read through this entire thread you'll see that I made a couple of mistakes here; if you continue reading you'll also see how I attempted to correct those mistakes.
In the final analysis, while I can't always provide a precise definition of abuse, I've been doing this long enough now that - most of the time - I know it when I see it.
Again, if you think I've made a mistake in a particular ruling, a polite FReep mail stating your objection(s) often works wonders. Sadly, unless I catch myself in time, calling my attention to what is perceived to be my mistake with a harsh comment on the thread will often result in whatever is the opposite of "wonders". A kind word turns away wrath, or however it's said in the myriad of Bible translations extant today.