I understood now also about the nuking! Thank you both!
However, there is something unusual about the Watchlist, as far as I can see... I'll detail below:
1. I started with the question how to unfollow topics where you previously posted. Why? Because I use "Show replies to your posts" and I observed that there are listed all the topics where you previously posted, in case new replies were posted in the respective threads. It seemed that if I write inside a topic, I'm automatically subscribed to any new replies created in the respective topic, fact reflected by showing that topic again and again under "Show new replies to your posts". And I didn't want to see some of the updated topics where I previously posted something. For example, I posted once in the "Freebitco.in win a Lamborghini Huracan" topic, asking how I can delete my account. I found out this thing is impossible, but since then, so many replies appear in that thread and I keep seeing it in the "Show Replies to your posts" link (which I used to use prior you mentioned me the Watchlist). I'm not interested in seeing again and again that new replies appeared in that topic, as the only reason I wrote inside it was to find out if I can delete my account from their site.
2. Afterwards, you mentioned the Watchlist, and initially I thought there appear also all the topics where I previously posted. I thought it works the same way as "Show new replies to your posts". Thus, when I clicked on Watchlist, I expected to see there all the topics where I posted at least once, fact which automatically subscribed me to them (take into consideration that, as I never used Watchlist before, I also never added a topic to my Watchlist). However, I don't see in the Watchlist all the topics where I previously posted, but only some of them. For example, the Freebitco.in topic doesn't appear in Watchlist, although it appears (when new replies are made) in the "Show new replies to your posts".
3. From 1. and 2. I suspect that either the respective two forum features work differently, or one of them is buggy (I think the Watchlist). Why do I think that Watchlist is buggy? Because I never used it until yesterday, but, however, there were several topics listed under my Watchlist. Nobody added them there; they appeared there at my first usage of Watchlist. So a possible bug is that Watchlist adds in a curious manner some topics, without user's action. Another one would be, in case Watch list is supposed to work exactly as "Show new replies to your posts", that it actually doesn't work the same way. Because in the "Show new replies to your posts" appear all the topics where you previously posted, in case others wrote new replies, but in Watchlist don't appear all the topics where you posted.
4. Of course, there is another possibility as well, meaning that I didn't understand yet how the Watchlist actually works
(and there are heavy chances to be this way).
But even so, if 4. is the correct answer, then I still don't know to unfollow a thread where you previously posted, to be more precise how to make that the respective topic won't be listed anymore under "Show new replies to your posts" (in case other users wrote inside those topics).
Also, why do I see some topics in the Watchlist if I never used it before?
Any idea about this behavior of the respective two forum features?