Yet security researchers have shown there are workarounds to all these methods.

Some hackers are now getting extremely clever about their methodologies. When the Federal Communications Commission was preparing to repeal net neutrality last year, there were 22 million comments posted on its site between April 2017 and October 2017, many of which expressed support for the move.

Jeff Kao, a data scientist, used a machine-learning algorithm to discover that 1.3 million comments were likely fakes posted by bots. Many comments appeared to be very convincing, with coherent and natural-sounding sentences, but it turned out that there were many duplicates of the same comments, subbing out a few words for synonyms.

“It was like Mad Libs,” he said. “If you read through different comments one by one, it’s hard to tell that some are from the same template. But if you use these machine learning algorithms, you can pick out some of these clusters.”

The F.C.C. said in a letter that it planned to re-engineer its comment system in light of the fakes.

What can I do?

For the issue of spoofed comments, there is a fairly simple solution: You can report them to the site’s owner, which will likely analyze and remove the fakes.

Other than that, don’t take web comments at face value. Mr. Kao said the lesson he learned was to always try to view comments in a wider context. Look at a commenter’s history of past posts, or fact-check any dubious claims or endorsements elsewhere on the web, he said.

But for truly offensive comments, the reality is that consumers have very little power to fight them. Tech companies like YouTube, Facebook and Twitter have published guidelines for what types of comments and material are allowed on their sites, and they provide tools for people to flag and report inappropriate content.

Yet once you report an offensive comment, it is typically up to tech companies to decide whether it threatens your safety or violates a law — and often harassers know exactly how offensive they can be without clearly breaking rules. Historically, tech companies have been conservative and fickle about removing inappropriate comments, largely to maintain their positions as neutral platforms where people can freely express themselves.