I don't think KPR's comment that "Kenny was Best, Law and Charlton rolled into one" is at all out of line or bad ****. It was just highlighted as a laughably uninformed post that shows his massive pro Liverpool and anti Man Utd bias. So Constcrepe just responded with a similar bit of banter poking fun at him for it. That, to me, is just banter and I have no problem with KPR or anyone else expressing genuine and inoffensive opinions, even if they are completely wrong to any reasonable person. But if they express those opinions on a debate and banter forum they have to be willing to be mocked mercilessly for it
Drawing the line is always a difficult thing, but I think most people can be reasonable about these things. You cross the line when you start joking about serious things like death, disease and so on. Fortunately, when that happens most people tend to walk away in disgust, or ask the mods to sort it out, rather than responding in kind. Much like when KPR was banned for talking about Best dying due to alcoholism - as far as I remember no Utd fans responded with anything along similar lines about Liverpool, and most Liverpool fans were just as disgusted.