I think you acted correctly. Like you, first of all I would ask the person what are his arguments to say bitcoin is a ponzi. Depending the answer I would have to reply accordingly the presented situation, what wouldn't be an issue anyway, since bitcoin itself has nothing to do with ponzi schemes. People who make such claims don't know what they are talking about, or they just want to badmouth bitcoin for some shady reason.
If the person isn't educated about bitcoin yet I try to explain and make them understand it, however, if I see the person is talking maliciously I don't waste any time arguing and just move ahead.