I had an interesting conversation with my niece this weekend. She quite regularly, along with her friends, calls people "retards" or concepts "retarded". She is no stranger to being bullied for her own significant SN. She truly did not understand where the term came from and has resolved not to use it. Obviously, she's young and therefore it's very different but the word has slipped into her lexicon so easily.
DH's work similarly uses a term (though not retard) to describe any difficult job and I have heard his colleagues freely use it. When I pointed out what the term actually meant, they were universally horrified that they used it so openly.
Perhaps 'spazz' is one that we presume DM knows shouldn't be used for comedic gain but does he know how loaded it is as a term? Does he mean offence to a certain group of people? That I think I can safely presume he doesn't. I think often the crime is ignorance as opposed to cruelty.
A kneejerk reaction and labelling somebody a gobshite for the singular use of a certain word isn't endeavouring to inform or educate or change the way we use language, it's reductive and presumptuous.
I won't villify DM for the single use of a word. If I was sure he understood the offence he inadvertantly caused yet still persisted, I'd be right there with you.
Clearly, I'm wittering. But I think there lies a problem in language usage not being challenged more openly. Perhaps DM could write a column about it. Or get him on for a webchat. Then we could talk about the issue instead of villifying the individual.