In my experience, trolls get away with trolling because the off topic rule gets enforced too strictly, the anti trolling rules don't get enforced enough and the response is often to just ignore them despite the lack of block feature.
Conversations drift into focusing on related elements that are still relevant to the topic but get shut down anyway. When this happens due to an argument involving a troll, it comes off like they don't really care if the actual content of the argument is still relevant to the thread topic and just want to stop it without acknowledging the trolling element that caused it to happen in the first place.
What's the point of having trolling as something explicitly listed as something to report if it never gets enforced? Sealioning is a form of trolling. You can give the benefit of the doubt if it's the first time that person does something like that but it it's a consistent pattern across multiple threads that multiple people have reported and been affected by, it's time to acknowledge that yes, they are indeed trolling and they should get warned to either stop or get banned from posting. Banning the trolls if warnings don't stop them is the most reliable way to deal with them as they can't troll anymore if they can't post. I don't just mean apparently only giving them a 3 day slap on the wrist, either. Repeat offenses should warrant longer bans. The whole reason this is a problem is because there aren't any actual consequences for what they're doing. This can be fixed.
I think a discussion on what's defined as off topic is important to have too. If someone makes a thread about a problem and discussing the cause and solution to that problem, discussing the nuances of that should count as a natural progression of the discussion. If you're too strict on topics and making new threads for every subcategory of what's relevant, it spreads the topic too thin and makes it hard to actually keep talking about it.
How to block a forum user
Is that a reply to me or OP? I'm referring to incidents where threads were derailed and ruined by trolls. Last time I saw it was a good few months ago. Any other incident you may be referencing is not what my post was about.
Short posts that are just antagonizing other posters or insulting groups of people are pretty straightforward removal. I've seen a few times where mods quoted removed posts to preserve the parts not breaking rules, which seems like a good compromize for when someone contributes to a thread but part of it has to go for flaming, etc.
The big issue is when a troll is trolling in a way that isn't enforced despite the pattern making it clear it's trolling rather than good faith discussion. Another issue is when off topic is too strict about valid points of discussion within a thread vs creating a new one or used to get out of actually calling out the trolling that brought the mod into the thread to begin with.
Even if a troll like that says something worth discussing, you'll never get anywhere because they're acting in bad faith. They repeat themselves, ignore your points and then the thread gets ruined. The mods just tell you to ignore them without actually punishing them for it but you can't because there's no block button. Not only do they not get punished but the lack of block enables them to keep being a nuisance.
If a thread gets ruined by a troll and all the stuff left to talk about is too off topic then why would someone feel motivated to actually make the new thread? The troll would probably just try to ruin that thread too. The bad forum experiences like that can be demotivating and discourage people from posting.
How to block a forum user