If by 'harmful' speech you mean conspiracies, I would argue they've always been around. It's not a new problem. People have always filled the gaps in their knowledge with unjustified beliefs.

However, your answer would be to deplatform those who would post such ideas. Do you honestly think that would make it better? Did deplatforming Alex Jones help people? It just galvanized those who believed there was a conspiracy and made him all the more popular.

How about we take on the hard task of trying to actually reach those who we believe have false ideas. For example, how do people respond to flat-earthers? Constant ridicule and laughter. The reason they believe in these theories is a lot more complex and nuanced than merely that they are 'stupid'.