When you decrease your culture it doesn't decrease a total amount of culture accumulated for the purposes of tourism. So, for example, if you want to get a boost to win a culture mission from a City-State, and then decrease it back the next turn, it will not decrease the total amout ever gained, so you can't "uncheat" it.
And considering there is no Player.
SetJONSCultureEverGenerated, only Player.
GetJONSCultureEverGenerated, and the increment to total culture is probably hardcoded in Player.SetJONSCulture, I guess it is not that easy to fix and would require some dll work. So the easiest way is to use CheatEngine. I am posting this just to let people know it is a thing.
How I found it:
I had a situation where my opponent probably used a Great Writer before the turn the quest ended, so he overtook me by a huge unknown margin (as I was leading the turn prior), and after a couple of reloads I had to give myself 10k culture instead of just a few hundreds (I assumed it was a few hundreds because my opponent's base Culture was higher, but I had a Golden Age for most of the quest, which ended 2-3 turns before quest ended), after which at the next turn I noticed a couple of random notifications about how the culture of other civs became less influential over mine, after which I investigated. I actually don't know if the AI did it on purpose or not, if the AI is trained to do so then I wouldn't cheat in such cases, so if anybody knows please tell me.
Actually, writing this made me realise you can try to get a little more culture by switching all you cities to Culture Process, so there would be no need to cheat. For some reason I never thought about this simple tactic, maybe because I memorized from vanilla Civ5 that you can't do anything to get a little more culture, as there was no Culture Process. And in VP I used it only in late game during Golden Ages (which is +20% culture AND +20% production) to get stupid amounts of culture very quickly (which, I now realise, can be used not only to unlock more Policies, but also to defend against someone's Tourism win, which is a very cool tactic).
A lot of realisations from my first comment on a forum, haha
P.S.: Also, does anybody know what JONS mean?