If people will throw at you some stupid arguments like, for example: bitcoin is a Ponzi - you have to know how Ponzi scheme works, explain difference, between Ponzi and Bitcoin's network etc.
Usually people who are anti bitcoin, have no idea what they are talking about. They repeat headlines of stupid media coverage heard somewhere.
Any logical arguments presented with confidence and logic is usually end of discussion.
Even gold is smuggled but still people never blame the metal and same goes with bitcoins. When we buy gold, there is no ID proof required and hence it's as good as gold if we compare the two. Media just wants to destroy the name of bitcoins.