The best way to handle trolls, depending on your preferences, is to either ignore them or report them as possible trolls.
It’s true that the system alerts the mods if five posters are ignoring the same person. But, in my experience (and I’ve been moderating another Discourse forum for a while now) posters who get ignored a lot usually aren’t trolls. Often, they are people with a minority view on some subject and an abrasive style. Sometimes they are posters who are unusually chatty. Sometimes they are rude and abusive posters – which is valuable for the mods, even though they aren’t trolls.
But i don’t think I’ve ever seen the mods recognize a troll based on posters ignoring someone. You should ignore posters based on your own preferences, not as a way to send a message to the mods.
If you think someone is trolling, report it. For that matter, if you think someone is rude and abusive, or otherwise violating community standards, please report them, and explain why. If you think there’s a pattern, it’s helpful to report a couple of posts and explain your reasoning.