The fallacy I hear used frequently is Personal Incredulity because people are comfortable with being ignorant. This looks like a person assuming or claiming something to not be true because they have trouble understanding it. For example, two people are discussing their gym routine, when one person goes “You use small weights?? You know they don’t actually work. The workouts aren’t hard, you’ll never see any gains. Weights have to be heavy!” To them, a workout is only effective if it is strenuous and results are noticeable. Because this is their understanding, of course they are going to reject the idea that light weights can make any kind of difference.