Updated: September 13, 2021
Behold, a new name of an adventure book. For 'tis an adventure. First, let me explain what this is all about. I love ArmA 3. It's the best first person shooter. No, let me rephrase it: the only really serious shooter out there, and if you feel like you want to grow some hair on your chest, you ought to play it. Of course, the gameplay can be enhanced via all sorts of Steam Workshop content bits - mods, scenarios, and alike. So far so good.
One day, I decided to do a little bit of housekeeping - prune some of the old scenarios I downloaded back in 2013-2014, some of which were buggy, abandoned or both. Normally, this is not an issue. Go to the workshop and hit Unsubscribe. The problem is, what do you do when the actual scenario (or whatever component) you want gone no longer exists on Steam? Aha. Hence, this article.
Brute force approach
As any honest ArmA-ist, I decided to go about my hard disk, and delete any reference of the mission by hand. ArmA 3 missions come as PBO files. Well, this hasn't changed since Operation Flashpoint really. What did change is the location of mission files, and the sheer complexity of the whole deal.
In the mission selection screen, click on (any) scenario to go to the Workshop. Unless the page ain't there ... P.S. Coast Defend is a very nice and fully working mission, good for clearing your chakras of work aggression.
Soon, I found myself digging through folder after folder. Some of the maps are saved in a very specific, esoteric folder inside the Steam installation directory. You can also find missions under My Documents. Also, under AppData/Local, in one or two places. There are also different locations for singleplayer and multiplayer missions. There be a mission cache, too. Plus your saves. Gets convoluted and painful. The worst thing, despite my rigor, one particular mission simply refused to go away.
Simpler approach
Then, I found a better, more elegant but also unexpected solution. In ArmA 3, when you want to select a mission to play, there's an option to check your subscribed content (all of it). Rather than exploring the selected element in the Workshop, you want the overview of all the different items you've ever subscribed to for the particular game. In this case, ArmA 3. If you think, trivial, it isn't. OK, click on View My Subscriptions.
This Visit My Subscriptions shows everything ... even deleted stuff.
Here, I found a long list of items, dating back to the very first days of my fun with this title, including quite a few items that I've already deleted by other methods (like the standard Unsubscribe in the Workshop). For some odd reason, Steam was listing those, too. It would appear there are two levels of subscribe. On the individual level, you can manage each item separately - subscribe/unsubscribe - through their individual Workshop pages, IF they exist. On the global level, you can completely "delete" these items, regardless of their current status in the Workshop, or if you've already unsubscribed from them individually and they no longer show in your game. Weird, but that is my observation and conclusion here.
Here, you can perma-unsubscribe.
And now, the conclusion ...
Here we go. I spent quite a bit of time reading various Steam game posts, and I realized that the content management is still a bit wild. One, different games don't necessarily handle deleted remote content well if there's a local copy. Two, the location of different workshop items on the local disk is a bit weird. Three, it seems that there are two separate ways to handle subscriptions. Four, I was able to get things done, and I can now rest.
I don't know if this problem affects other games in the same manner. For ArmA 3, at least, the solution is to go down the global route. But that also means you will have no more "history" of the particular item, so if you want to re-subscribe in the future (if it exists), then you will need to find the content manually. Anyway, no need to delete anything by hand, no need to trawl through folders and potentially ruin a game configuration. Now, off I go. It's time to put some fresh hair on me chest.
Cheers.