This doesn't seem practical. People will make mistakes. For that matter, the autistic employee may well be the one making the mistake, and the things he's pointing out in his emails may not always be errors.

@DaveG thanks for your comment. I agree that mistakes happen. I also think that mistakes are needed to learn. My suggestion is to improve processes so that mistakes don't matter. Given that the employee is unfireable, and this is in the SE of The Workplace rather than Interpersonal Skills, I feel it's the only way forward here. (From the downvotes it seems there's something I've missed :-))

It may not sound practical, but it is all you CAN do. He has a medical exuse and firing him is off the line. Rock, meet hard place. Plus he is factually right - and if that is cuasing problems... well. Not HIS problem (btw,. a chceckin causing problems is a serious issue in the infrastructure - no build gates installed).

Would other downvoters care to explain what they think I'm missing here? The employee in question is doubtlessly aware that they can't/won't be fired. That makes them untouchable. Explaining the problem to them hasn't worked. What magical words exist to compel someone to change?

Take a look at Richard U's answer for a practical alternative.

@TomTom autism is not a suit of armor. Reasonable accommodation under the ADA does not include allowing someone to be abrasive.

@DaveG Indeed! I upvoted it the second after I read it :-)

@Richard U it's not because of autism he can't be fired, it's because of Nordic employment regulations/customs, according to the OP. Your answer is fantastic by the way.

@AaronF sorry, I deleted my comment.

You cannot change this behaviour unless you're willing to resort to some sort of disciplinary action. This comes out of nowhere. I think it's nonsense. Why? See the other answers.

@RichardU "accommodation under the ADA" are irrelevant. Did you read the little note about "We are in a Nordic coutnry" - that is Denmark, Norway, Sweden etc. - US laws generally do not apply there and they are WAY more socialistic inclined.

@TomTom I refer to the ADA so often it's a force of habit. Regardless, the point remains. You don't have to accommodate someone being a disruption.

@TomTom yeah, I still have family in Germany and an understanding of the laws of Denmark. Like I said, I've run afoul of the ADA so often, it's a knee jerk reaction for me to mention it.

The first line of your answer is quite obviously wrong, as you can see when you look at any of the proper answers.

@gnasher729 the OP clearly stated they won't fire the employee and that many people have already spoken to the employee. Given these pre-requisites, I didn't feel that an answer that boils down to "speak to them again" would be sufficient for the OP. It remains to be seen which, if any, of these answers is 'right' or 'wrong' for this particular situation. We'll only know for sure if the OP decides to edit in an update one day.

@reinierpost None of the other answers explain why you think it's nonsense. Perhaps you could have a go? Disciplinary action, or at least the threat of it, is a common tool, and often the only real power that an employer has (from a European perspective).

@Aaron F The other answers provide recommendations on how to act in other ways than taking disciplinary action (although some mention it as a last recourse). Flatly denying that any such thing may work is not a good answer by itself, I think; it requires some motivation.

@reinierpost thanks for replying :-) I think I could have structured my answer better. The big bold "you can't!" probably detracts from my qualifier "you won't fire them, and you've already tried talking to them". That's why I suggested to focus on processes - I felt it was better to suggest an alternative to work around the issue, than to suggest "I know you've already tried talking to them, but have you tried talking to them like this ?". I prefer RichardU's answer to my own; but I still think the OP's company need to sort out their deployment processes so that Mark can't do any more harm!

This really ought to say, "I don't know what you can do" instead of "there's nothing you can do." But then that means... why bother answering?

@ElysianFields on the contrary. Rather than ignore what the OP has already tried to do, like many other answers which suggest talking to the employee yet again; I suggest they try a different approach, one that will reduce or remove the potential for such problems to ever happen again. This is the Workplace SE, after all, and not Interpersonal Skills.

@AaronF Being an effective employee/manager/influencer requires interpersonal skills...