For example, you could continue this by closing/deleting of every hardware question, saying that a professional system administrator doesn't make his hands dirty with dusty server parts.
Then, you could continue by closing every software raid question, because a professional system administrator working environment uses a shared or network-attached storage. Using your own Linux server with software raid is not professional, could you say, of course I suggest to use some euphemism for that (for example: "we can't give the required support which it would deserve").
The next phase would be the closing and deleting of every question involving any web interface of any device. The reason would be, that they are essentially control panels, which is also forbidden already.
Of course, Windows servers and generally, Windows-based corporate networks shouldn't also let out from the destruction. Unfortunately, I don't know enough well these technologies to suggest my creative ideas, but I think similar reasonings would also work. For example, any question involving scripting and AD could be closed saying that programming questions are offtopic.
Most questions regarding the cloud could be also closed, saying that they don't involve actual system administration, with the exception if you are one of the sysadms of it (but in this case, you won't come here with your questions).
Questions involving the basic IP infrastructure could be also closed as un-professional. For a professional system administrator couldn't be a problem, for example, how the dns system works. There could be another reason, saying these questions are relating to routers and not to servers. This would make destructible every question regarding cisco devices and routing protocols.
Questions involving file versioning software could be also closed as they belong to software development and not to the professional system administration.
There could be yet a little problem. The CMs probably won't be happy. But fortunately, if there are 5 3k+ users working on a consistent, well-reasoned way, it will be very hard for them to argument, that it is not a community decision. Minor clashes are possible, for example revoked moderator permissions or some months in the cage, so the team doing the destruction should have some backup members, too.
Most of the CMs probably doesn't have a professional system administrator background, but they have probably a good general image from the sysadm work. This is exactly the knowledge level, where it will be probably a very strong argument if you say, that you are focusing now for the quality, and this requires the hard filtering. Your argumentation should target as if you wanted to reach some unbelievable, idealistic goal which will result a much better site.
Another focal point is that never, nobody should destroy a lot of questions at once. It would be much better if the destruction happened in small steps, and you should show always a consistent face to the CMs. You should also state, "we destroy only crap". It is very important, because the CMs sometimes think that 5 enough determined 3k+ users equals the community.
Of course, there will be always questions whose destruction won't be easily explainable. The strategy would be in their case, that find common points in them, and then create a reason for that point. The usage of the words "non-standard", "not professional" or "not supported" are always very helpful.
I think, in the case of the last some thousand questions there will be another serious problems. The SE surely won't allow to completely empty the site, it doesn't matter how well are you argumenting for that.
I think, these last some thousands of questions should remain. After that, it will be enough if only the new users would be consistently harassed, bullied - for example, because they don't know all of the relevant meta posts regarding the topic of the site. So, their questions should be closed as offtopic. In some month, maybe in some years, the site will be slowly get out from the google cache, the number of the new users will drop, and the site-suicide will be finally
Extension: It seems, we can finally delete everything about VirtualBox! This wonderful idea can be applied anywhere: only we should switch "uncommon" software to "unprofessional". Of course, nearly all emulation and virtualization solution could be also deleted on this reasoning, except hyperv and vmware.