This has also annoyed me in the past with my "main" Sim, who is a Loner. I'd used cheats every now and then to reset her Mischief skill to 0 every now and then to try and get her to stop randomly trolling the forums, but eventually I just gave up because it was so relentless. I'll still cancel the interaction for her when I can, but am not as strict about it
If it's any comfort, though, there IS a good-natured type of internet trolling that people refer to as "do no harm" trolling. Instead of trying to make people angry and upset, "do no harm" trolls try to perplex people by making super foolish and/or harmlessly absurd posts, with the ultimate goal of making other people laugh at the ridiculousness. Nowadays, when I see a more good-natured Sim choosing to "troll teh forums", I'd like to imagine that's what they're doing
That being said, I don't think I've ever let a Sim troll for very long, so I don't know if there's a moodlet they get from it or anything. If they have something that specifically confirms that they're doing the mean kind of trolling, then welp. There goes that
By the way, "Insult", "Jeer", and "Yell At" are all actually Mean interactions IIRC, not Mischief
