What's too far in fandom, then?

So first there was Anybody But Sara (ABS for short) and now there's 'Anybody But ABS' around.

Then there was the old, old 'Catherine is a slut!' 'No, she isn't!'

Greggsgirl thinks that all strippers are sluts, apparently:
Ok! This video would be really great if it was Gregg in there! But anyway Catherine is such a HO! She thinks that evey guy wants her because she was a stripper! Who really even cares! That was the only job she could get were a guy doesn't care what your face looks like cuz if they did care she would have starved! In my opinion she otta go back to that job! Cuz she still acts like a total SLUTT!

ericasj is not particulary amused:
Bashing someone like that ain't cool at all. Hold it until you're in your forties and look as good as she does now. And face it, Catherine is the leading female of this show and you won't see her leave anytime soon.

But we shouldn't listen to her, as she's just out to get in Catherine's pants:
Ok! Whateva! In my opinion you r just plain retarded ericasj because i find it strange to think that you probably a woman yourself thinks that ho (i mean catherine) and i quote "looks good"! yeah if your a lesbo!

And after these two examples of things getting personal, here's my question - what is too far in disagreements online? Where do you draw the line at Fighting For Your Ship/Favourite Character/Version of Canon? How easy is it to form little vendettas online and drive them too far?

All too easy, I have a sneaking suspicion. CSI fandom is not alone - Harry Potter fandom has had some infamous disagreements that started over ships and got way personal, Doctor Who fandom has had different views on Rose get somewhat nasty... I'm sure you can think of examples from other fandoms. And giving the bias of the brain, seeing everything subjective, there's not going to be agreement on everything in a fandom. Debates will happen, which is mostly good.

But here's two things I think is taking things too far.

1) Sexual slurs. Abhor them. Whether against fictional characters or online personas, the fact that it so, so often gets throw around about women by women just leaves me feeling depressed. Why is someone's sexual history a weapon to be used against them? Not on. Not on at all. And a subsection under this - going after someone's looks. Because 'ugly' and 'fat' are disqualifications for your opinions mattering, apparently, and totally valid reasons for character bashing. Yeeeep. Beauty is not in the eye of the beholder at all, it seems.

2) Using real life stuff against someone online. When the disagreement is online, keep it online. No need to make the playing field muddy.

That's two of mine. What are yours? And have you ever gone too far and only realised it later? (I know I have.) How much does peer support play into behaviour in fandom? And if The Other Side has crossed the line, is it okay to cross it as well? I'm curious what people think, so... Opinions, please?


(P.S For Sondre Lerche fans - sneak exclusive preview of his entire album!)