You can do this by creating a counterargument to any reason or example that they give that justifies their claims.
For instance, using the popular saying that the use of crypto currency promotes terrorism funding and every sort of illegal activity, you can give an instance of where fiats have been used for such cases, letting them know that crypto is not the problem but the people using it are.
One big thing is education, once people get enlightened the misconceptions would begin to die off. it's just a matter of time bitcoin is already on this way to make a more big difference that could lead to people regretting their actions of being stubborn towards it.