Insofar as AnCaps like Rothbard claim to know what an ideal society would look like (it would/wouldn't have intellectual property, etc.), they contradict themselves. However, it is entirely unnecessary for an anti-statist to claim they know how society should ideally operate; consistent AnCaps understand that the market (the natural order of society) will determine everything.

Great debate on this exact point:

http://wwww.qjae.org/community/forums/p/10277/273307.aspx#273307