I'd say the most important dividing line would be ambiguity.
There are a lot of grey areas... but acronyms are inherently ambiguous. You cannot maintain the same information density when you condense arbitrarily long words into a single letter.
But there's another dividing line... The meaning of words might change over time... but over the course of a small amount of time, that change is very small. The meaning of one word won't be altered completely by tomorrow. And that in turn means that words that were once used as slurs still carry that connotation simply by their history.
An acronym on the other hand doesn't really have a histoy in that sense, and also is not limited to small changes in meaning at a time. If by tomorrow a live-changing invention is made, that - unfortunate as it is - consists of three words, each beginning with a K, the acronym KKK suddenly has a different meaning that is completely off from the original meaning.
I won't voice my opinion on whether I think 'KKK' in particular is acceptable as a clan-tag, though... at least not when it comes to clans on the NA-server, since I lack the cultural "background" to judge that. I simply don't have that strong of an association connected to those letters.
Edit: The only scenario for me that I can imagine coming close to KKK for americans, would be someone using SS or maybe HH as a clan-tag... but even if it was less ambiguous, like HEIL, or something like that, I don't think I would report it.
It might make me a bit uncomfortable, but unless it's 100% unambigous that the intention of the tag was to mimic that acronym, I wouldn't really think much of it.